Backdoor flush with 23s, 160bb deep NL100 - river bet size?

[tubasteve]

Full Tilt Poker, $0.50/$1 NL Hold'em Cash Game, 6 Players
LegoPoker Hand History Converter

SB: $99
Hero (BB): $161.40
UTG: $32.65
MP: $218.15
CO: $39.90
BTN: $139

Pre-Flop: 2[img]/images/graemlins/heart.gif[/img] 3[img]/images/graemlins/heart.gif[/img] dealt to Hero (BB)
UTG folds, MP calls $1, CO calls $1, 2 folds, Hero checks

Flop: ($3.50) 6[img]/images/graemlins/heart.gif[/img] 4 T (3 Players)
Hero checks, MP checks, CO checks

Turn: ($3.50) J[img]/images/graemlins/heart.gif[/img] (3 Players)
<font color="red">Hero bets $2.50</font>, MP calls $2.50, CO folds

River: ($8.50) 9[img]/images/graemlins/heart.gif[/img] (2 Players)
<font color="red">Hero bets $6.50</font>, <font color="red">MP raises to $13</font>, Hero...

I'm definitely reraising here, my question is, how much? Villain is the worst player at the table, a 36/9/.3.