1,500 points seems to be the most common -- enough points to field "interesting" things, yet low enough to allow the game to wrap up in under 3 hours. I've found that much bigger 1 on 1 games can take forever, especially when it's clear you're going to lose early on.
I'd suggest building your force to support the most common game sizes: 600, 1,000 and 1,500 points. After that, you usually have enough to be quite flexible. For new players, it's a bummer to have put together a bunch of stuff and realize you can't field a legal list with it, so follow the book guidelines as best you can.
As far as force selection, I can't help because I play Soviets!
- Bob