≥70" would be awfully nice, but only for a steep discount
feet that are absolutely no wider than 36" (really should be <32" wide)
resolution — I really don't care if it's 4K, but 1080p is a minimum
smart TV — it might be nice if Android TV were built-in, but it shouldn't be a big deal to plug in a separate Chromecast, right? (TODO — is there a serious advantage to having the smart-TV functionality built-in? Such as being able to change the source using voice commands?)
number of inputs — the more the better, obviously, though it seems very likely that we'll buy a good HDMI switcher[2] so the number of inputs may be less crucial