A typical rule of thumb is to open raise (i.e., if you're first to enter the pot) 4x the BB from any position. If you're entering a pot behind limpers, add 1BB for each limper. So, on the button with two limpers in front of you, you'd raise to 6xBB.
You'll need to tailor this to your style of play, the stakes you play, and the current table dynamics.
There are other things to take account of later (position, stealing frequency, and villain types), but you'll probably want to start with something like this basic/simplistic model.
Oh, to D'Wilius' point, I'm specifically talking about cash games and not MTT's/SNG's.