[ArtVandelay]

Assuming you're new to big-bet poker, I recommend always betting/raising exactly the pot, or close enough to it. You give away no information this way, and you're never far wrong. Also, I disagree with Ignatius that if a pot bet/raise is 40% of your stack you should push all-in. That would have you betting 2.5 times the pot in some situations, which is generally wrong. If the pot bet/raise would be maybe 80% of your stack then the last 20% isn't far wrong (especially if you don't want to have a tough decision on the next street when certain cards hit), but I think even up to 60% or 70% of your stack you're probably fine just going pot as normal.

Once you've played lots of hours and learned some of the elements unique to big bet then you should start playing around with different bet sizes. Another common one is half-pot, and after that anything goes. You can do clever things like bet an ammount on the flop so that on the turn you have exactly one pot-bet left in your stack, or you can bet an amount giving your opponent exact odds for a flush draw. When you raise pre-flop and it looks like the flop missed you, you can bet small when you actually flopped big trying to induce a bluff. When you flopped trips from the blinds and it's check around on the flop you can "steal" it with a big bet only to crush the guy who tries to re-steal with a large raise. As expected there are innumerable possibilities, and it's one of the funnest, most creative aspects of the game.

However, I STRONGLY recommend that when starting out you confine yourself to always bet/raise the pot.