This looks like a simple question, but it's actually quite complicated. I assume we're talking about the flop?
Taking the question literally, in cash games my default bet size on the flop when I bet the flop is about 2/3 pot - however, there are a lot of reasons that the betsize should change, based on preflop action, the board, the villain(s) playing the hand, my perception of villain's strength, and villain's perception of me/my strength. I never bet less than half pot when I bet the flop, and very rarely bet full pot (or more) on the flop.
I'm not a very good MTT player, but I bet much smaller on the flop (usually just over half pot) in MTT's because of the escalating blind/stack ratio. Someone who's better at MTT's might have a different suggestion.