Party 10/20NL, missed OES&Flush draw OOP on river, correct bluff size?

[IHateCats]

Party Poker No-Limit Hold'em, $20 BB (8 handed) FTR converter on zerodivide.cx

MP1 ($1980)
MP2 ($2586.14)
CO ($1842)
Button ($2147)
Hero ($2067)
BB ($605)
UTG ($1775)
UTG+1 ($2260)

Preflop: Hero is SB with 7[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/heart.gif[/img].
i]1 fold[/i], UTG+1 raises to $65, 2 folds, CO calls $65, 1 fold, Hero calls $55, BB calls $45.

Flop: ($260) 2[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/spade.gif[/img] (4 players)

Hero bets $175</font>, BB folds, UTG+1 calls $175, CO folds.

Turn: ($610) 9[img]/images/graemlins/diamond.gif[/img] (2 players)</font>
Hero bets $350 , UTG+1 calls $350.

River: ($1310) 6[img]/images/graemlins/club.gif[/img] 2 players)

Opponent is relatively new to me, niether ultra tight or Lag, I've been somewhat tight the 3 rounds since I sat down at this table. Obviously I've missed but that's an ideal scare card for me. What is the correct ammount to bluff here to blow out what I assume is an overpair, yet lose the minimum if he's the type who just can't let AA go. I'd consider a rebluff very, very unlikely under the circumstances if I open lead and he had AhKh or something similar.