A "normal" buy-in is 100BBs
More than that, it's deep, less than that it's shallow
One could argue that the buy-in, among socially related people, should be standard 100BBs, or half the big stack, eventually.
If you go for variable min/max, I guess the range should be 60 to 120 or 150 BBs.
For games among socially related people which are meant to be "friendly" (a ludicrous term for a violent game like poker) you could also establish a total cap per player per night/session.
So that if an otherwise nice guy comes to a .25/.50 game with $ 1K in his pocket, he won't be allowed to table all that, allowing him to play like a maniac, eventually unpunished (for the sole reason that other players will NOT have the liquidity to see him to his demise)