I guess in theory (and from an EV standpoint), you should make as large a bet as he will call, assuming that you are not giving him pot odds. If he folds, that's okay too.

I think always giving your opponent credit for the flush is a leak unless several people saw the flop. You need to call down occasionally to discourage the attentive players from bluffing you. Admittedly, this is probably not a major concern at mant Party $25 NL tables

Anyway, if it's not a leak, it's fuzzy thinking. If you say you won't push because you're afraid he'll call, then he'll certainly call a pot-sized bet. If you're going to lay your hand down if the flush completes, then you're willing to lose if it hits, regardless of whether he's got the goods. This effectively gives him more outs. If your goal is to lose the least when the flush draw hits, you should check. If your goal is to win the most, then you should bet as much as he will call.

Bottom line - the reason you bet the pot rather than pushing is that you *want* the flush draw to call, not because you are *afraid* he'll call.