Technically anything over 55 inches is considered a public performance. It's leftover from an old law:

http://www.law.cornell.edu/uscode/text/17/110

The NFL holds copyright in its broadcasts of the games, and copyright law gives the copyright owner the exclusive right of public performance and public display. However, the Copyright Act provides an exemption allowing for "performances" of radio or broadcast/cable/satellite TV without violating copyright.

There are some fairly obvious restrictions to the exemption (for example, you can't directly charge people to see the broadcast), but also some surprisingly specific ones, and there's where you start getting into TV screen sizes:

> if the performance or display is by audiovisual means, any visual portion of the performance or display is communicated by means of a total of not more than 4 audiovisual devices, of which not more than one audiovisual device is located in any 1 room, and no such audiovisual device has a diagonal screen size greater than 55 inches, and any audio portion of the performance or display is communicated by means of a total of not more than 6 loudspeakers, of which not more than 4 loudspeakers are located in any 1 room or adjoining outdoor space

(Note: the above are rules for bars/restaurants. There are different rules for other establishments, and much more permissive ones designed to cover private homes and parties)