How big a TV should you buy? 37? 42? 50? 65? 90? There's a TV in nearly every size you can want, and at nearly every budget.
As long as you're not limited by a cabinet or entertainment center, you can probably get a bigger TV than you're figuring. Possibly, a lot bigger.
Here's how to figure out how big you can go.
The short answer: as big as you can afford. The longer answer depends on your room, your seating distance, your eyes, and the acceptance of any cohabitating co-deciders.
There is a direct correlation between recommended screen size and seating distance. The farther away you sit, obviously, the smaller your TV appears. The ideal is to have a screen that fills a certain amount of your field of view, though how much is "ideal" is up for debate.
THX recommends, for example, a viewing angle of 40 degrees, to create "an immersive cinematic experience." What does 40 degrees mean to those of us who love math in theory but not in practice? Skip the protractor, and multiply your seating distance (in inches or centimeters) by 0.84. This gets you the recommended screen diagonal.
For example, most people sit about 9 feet (108 inches) from their TV, so THX recommends a screen size of around 90 inches diagonal for that distance. That means the 55-inch you're looking at is not "too big," at least as far as THX is concerned. They realize, though, that not everyone has the space (or desire) for a TV that big, so they also recommend something in the 60-inch range for seating distances up to 9 feet.
SMPTE recommends 30 degrees, obviously quite a bit smaller. To match their recommendation, multiply your seating distance by 0.625. So given our 9-foot example, that means a TV around 68 inches (so a 65- or 70-inch model would work).
CNET, when this topic was covered last, recommended no closer than 1.5 times the screen diagonal, or 0.667 times your seating distance. This allows for a slightly larger TV than the SMPTE recommendation, with a "maximum" screen size of 72 inches (again, 70-inch models are the closest) for a 9-foot viewing distance.
While these are good guidelines, I think they need to be taken as the recommendations they are, not any sort of steadfast rules. It's far too easy to get caught up in a numbers game when the reality is far more complex. There are additional factors in play than this-times-that-means-this would seem.
Beyond the "rules"
Two factors come into play when you're talking about maximum screen size: resolution and room domination. Most TVs now are 1080p, and that's a lot of pixels. Even at the largest flat panel screen sizes available today, you're not going to see pixels sitting at 9 feet. If the TV (or projector) isn't very good, you might see other artifacts like video noise or dithering, but you're not likely to see pixels. If you're sitting closer to such a large TV, pixels may start getting visible, but you'd have to be sitting awfully close. This is one of the reasons I said Ultra HD 4K TVs are stupid, and why there are at least three TV improvements more worthwhile than Ultra HD 4K, and there's more math in that both articles about what resolution the human eye can actually see.
The short version is: extremely large TVs may not look quite as good when you're sitting really close to them, as you're seeing more than they were designed for you to see. That isn't to say they'll be "unwatchable" close-up, they just may not look as good as if you were sitting farther away. This is especially true if you're not always watching a pristine source like Blu-ray. Check out When HD isn't HD for more on the lesser sources that will look really bad on a huge TV.
The other factor, room domination, is completely subjective. How big does a TV have to get in your room before it becomes the only thing in the room (either figuratively, or literally).
If you have any doubts, I'd recommend taping off or cutting out cardboard in the size of the TV you're thinking about, and seeing how it fares in your room. Know that once it's actually in there, it will be way more awesome than cardboard, and likely seem way, way bigger. It certainly depends on your room, decor, and overall spousal tolerance/enthusiasm.
If you think you might be sitting close enough to see pixels (or are curious if you are), check out this 4K Calculator and put in your viewing distance, screen size, and your vision (20/20, etc).
Beyond the beyond: What's possible
I'll be honest, I don't subscribe to any of the established "rules" for viewing distance and screen size. I think the SMPTE and the lesser THX numbers are too TV-biased. I think they vastly underestimate what's easily possible with modern technology, for those that want more.
I sit 9 feet from a 102-inch screen. That's just the 16x9 portion. The full screen is 2.35:1, and 128 inches diagonal. With a 1080p projector, I can just barely make out pixels when I expand a 1080p projector to the full width of the screen. Watching TV this size is addictive, and I love it.
I mention this as proof you can go much larger than most people figure is possible.
Do you want to? Well, that's an entirely different question. I find the larger screen sizes easier on the eyes, as more of your field of view is taken up with the roughly uniform brightness of the screen. In an otherwise dark room, your pupils are more naturally closed to the amount of light thanks to the big screen.
Conversely, I find watching a small screen in a dark room more fatiguing, as your pupils are more open (because of the dark room) with this one annoying pinprick of bright light (the TV). Many people complain about headaches when they watch TV in a dark room. One possible cause is the 50+ footlamberts from a TV (or more with LCDs) taking up a tiny fraction of your field of view. Think about when someone shines a flashlight in your eyes when you've been in the dark for an hour. With a projector, you've got 25 ftL or so over a huge swath of your vision.
True, bias lights, leaving room lights on, and turning down an LCD's backlight can minimize fatigue as well or better than a big screen, but I like watching TV in a dark room. To each their own.
Of course, the ultimate decision is one of personal preference. My goal here was to point out a rough idea of what is possible and/or recommended. For me, I would always err on the side of "too big." My opinion is that a 42-inch TV is too small for most rooms. That's not to say I think everyone should get a 102-inch screen, but the reality is a 42-inch flat panel is not really appreciably larger than the 36-inch CRTs of the old days (different aspect ratios notwithstanding). With great 50-inch TVs available for $500, that's where I'd look at first if I were shopping for a TV.
If you want to go really big, like an 85-inch TV or projection, only then should you think about 4K (unless you're sitting really close, like under 6 feet). Since there's still a lot of potential issues with HDMI 2.0 adoption, I'd be wary of 4K at all right now.
But like I said, that's me. What made you decide to get the TV size you bought?
Got a question for Geoff? First, check out all the other articles he's written on topics like HDMI cables, LED LCD vs. plasma, Active vs Passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you which TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.