Nut flush river bet size on a dryish board

This is a weekday evening 1/2 game. 8-handed.

UTG (loose passive typical Asian fish) opened 6.

Hero saw A4ss from UTG+1 and we flat.

UTG+2 (main V, white guy 20+, as handsome as a model) flat. He might have some overplay - we've seen him btn 3! with K7s in an earlier pot and gave up after c-betting an Ace high flop.

One LP flat.

BTN min 3! to 12 (I never quite understand why would anyone min 3 bet! so please share your thoughts if you've seen this more than me)

UTG folded, hero called, main V and LP called. 4-handed.

Flop (60) 2♣5♥8♠
All checked to the btn, btn bet 15.
hero called, V called from my direct left, LP folded.

3-way to see the turn.

Turn (105) 6♠
We now picked up an ace flush draw on top of the gut-shot.
We led for 75.
V tank called. BTN got out of the way.

River (255) J♠
V was the effective stack with about 300; we had him covered.

What range do we put V in and how big shall we bet?