Reader Lourdes asks: "I've chosen a flat screen TV for my 13 x 20 livingroom, but I'm not sure what screen size to get. What do you think would look best with my 77" media cabinet...? 37", 40", 42"?"
Well Lourdes, you have come to the right place...
First of all, you also mentioned that: "I'm not an avid TV watcher, but would like to have a decent sized TV for when guests come over for movie night; my media cabinet is pushed up against a 20 ft. wall and there is nothing on either side, except speakers on stands; and most importantly, I'm more concerned with having a well proportioned livingroom, than having a killer home theatre set up."
From what you say here, it sounds like your tv stand is against the long wall which gives you more options as far as size. You have also chosen a range of TV sizes that are not too far apart from each other so you have a higher chance of success.
Now a factor which you may not have been considering: viewing distance. The chart on the left describes optimum viewing distances that apply to different TV sizes. You haven't mentioned how far away you will be sitting, but that may change over the life of the TV anyway (moving furniture around and so forth.) Using this as a guide, you will want the larger TV since it will just suffice for an estimated 8' viewing distance and wont quite be enough for a 10'. HA! Rules... Who needs 'em? This is part of how manufacturers get you to spend more but you have more considerations to factor in...
A word on proportions: Your cabinet is 77" wide so the TV is going to have at least 17.5 inches on either side, no matter what you buy. It wont overpower your room because the stand itself will be more imposing. In the end your proportions of table to stand will be this (this is a proportionally correct Photoshopping of the actual stand and TV):
Advice: 42" DO IT!