Gotta figure out your opponent's:
a) likely holdings
b) calling and folding tendencies
c) perception of your range
Generally speaking I like to bet the flop with a monster. People will tend to peel pretty wide - over cards, draws, top pair, middle pair, pocket pair, sometimes even bottom pair.
There are weird spots though. Like you've got JT, OOP, and flop comes 789. If my opponent seemed aggressive pre-flop and appears to be a thinking player I like a min-donk bet. It gives you the appearance of being on a draw, rather than having a made hand.
If you think your opponent has a very strong holding like an overpair, a pot sized bet can be good because it looks like a protection bet rather than a monster.
There's a million ways it can go.
Ideally though, if you have a speculative hand you are in position and can react to your opponent's play.