555 - how much to raise?

PokerStars No-Limit Hold'em, $0.25 BB (8 handed)

UTG ($7.90)
UTG+1 ($13.40)
MP1 ($6.70)
MP2 ($16.90)
CO ($8.85)
Staresy ($20.55)
SB ($10.30)
BB ($43.50)

Preflop: Staresy is Button with 5:heart:, 5:diamond:. UTG+1 posts a blind of $0.25.
UTG calls $0.25, UTG+1 (poster) checks, MP1 calls $0.25, MP2 raises to $0.75, CO calls $0.75, Staresy calls $0.75, 2 folds, UTG calls $0.50, UTG+1 folds, MP1 folds.

Flop: ($3.85) 9:spade:, K:club:, 5:spade: (4 players)
UTG bets $0.5, MP2 calls $0.50, CO calls $0.50, Staresy raises to ?????

Pot's $5-ish, flush draw's out, I pot this one with four players in the hand, and hope one of them likes his draw too much.