two pair, river bet size

I imagined villain had KQ, maybe a draw, unlikely the set. What size river bet you like?
Full Tilt No-Limit Hold'em, $0.25 BB (9 handed) Full-Tilt Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver Cards)

MP2 ($24.40)
Hero ($23.90)
CO ($30.70)
Button ($37.25)
SB ($24.75)
BB ($15.50)
UTG ($28.10)
UTG+1 ($27.10)
MP1 ($25)

Preflop: Hero is MP3 with http://www.flopturnriver.com/phpBB2/.../smiles/kh.gif, http://www.flopturnriver.com/phpBB2/.../smiles/2h.gif.
3 folds, MP2 calls $0.25, Hero raises to $0.5, 4 folds, MP2 calls $0.25.

Flop: ($1.35) http://www.flopturnriver.com/phpBB2/.../smiles/ks.gif, http://www.flopturnriver.com/phpBB2/.../smiles/8s.gif, http://www.flopturnriver.com/phpBB2/.../smiles/2d.gif (2 players)
MP2 bets $0.5, Hero raises to $1.5, MP2 calls $1.

Turn: ($4.35) http://www.flopturnriver.com/phpBB2/.../smiles/7c.gif (2 players)
MP2 checks, Hero bets $3.75, MP2 calls $3.75.

River: ($11.85) http://www.flopturnriver.com/phpBB2/.../smiles/6d.gif (2 players)
MP2 checks, Hero bets ?