Bet sizing

You could go smaller, because you are targeting hi non flush hands to fold. (at least you should ) Because BvB and at PLO2 and with 4 spades on the board, and with no blockers at all there is no way that anyone will fold a flush on the river. AA/QQ is unlikely since he did not raise otf. He most likely has 2 pairs so around 14-15 BB seems good for me. But i'm not sure I would barrel this turn anyway.
I checked with Pokerjuice:
(the names and positions are not yours but it does not count)
Assuming he continues on the turn with any 2 pair or better, and raises AQ+ on the flop he will continue 57% of the time, so your bet is definetely profitable.

On the river I would give up. His range is heavy on flushes, and he may call you with 2 pairs some of the time

If you have the PJ file still available, can you check how often he has the flush on the river?

The problem is, that I used an other HH file and modified it. And there was no river there.. But I think I can check it out if I manage to learn game builder module.

now I have it.
Assuming he calls the turn with 78+ he will have a flush on the river 50,93%. assumng he does slowplay the nut flush on the turn: 58,81%. Assuming he only calls the turn with a8+ (which is far more likely than the 78+ IMO) than he will have a flush 76,1% of the time.