Phil Gordon wrote (I'm paraphrasing) -
Every time you put $ into a pot, you should have a goal and know what your objective is - is it to thin the field? - is to bluff or steal? - is it to build a bigger pot to take down later? - is it to offer him horrible odds to draw? - is it to draw to your hand because your opponents have kept it cheap enough to justify a call?

The size of your bet will be determined by your goal, modified by your understanding of your opponent.