I would not worry to much about it. Most emulated games I've seen, don't go beyond 300 pixles for horizontal res.
Most decent TV sets with 25 or 25" diagonals have around 600 total horizontal TV lines of res except very cheap ones probably.
Vertical resolution for all NTSC TV's (unless they are defective, very very bad or very small screens) is fixed at 480 scan lines because that is what the NTSC standard dictates. High definitinion sets are another story altogether.
If you want to see the resolution for each game MAME emulates, use the -listinfo parameter or use a frontend that will show it to you like EmuLoader.
If you have further questions, feel free to e-mail me.
Regards,
Manuel