It's a good starting point, at least regarding the resolution part of the equation. You won't benefit from the extra resolution of a 1080p panel at that distance.
The other, arguably more important part though, is the overall PQ (picture quality). In this regard, a really bad 1080p might not be as good as a really great 720p HDTV. At this juncture you should not be buying a 720p HDTV at all IMNSHO. Read through all sorts of ratings to figure out the drawbacks of each to see if one suits your needs. Understand, some of the panels have narrow viewing angles, which could affect your enjoyment when seated slightly at the sides. Others have washed out details/poor black performance/color issues. Short of testing each one out at home, you have to thoroughly mull through the user/review feedback. Good starting places are rtings.com, as well as Amazon.
Keep in mind that the larger the panel size, the easier it is to make out imperfections in various signals from different inputs. IOW, standard definition can really look bad, regardless of what type of upscaling is going on, etc. Trial and error unfortunately.