1/2 NL.
UT Open 10$, CO&BU call, I squeeze to 60$ AcKc from BB, UTG calls, 2folds. Eff stacks are 550$ . UTG is early 20's WG, who lost mayor portion of a big stack he build before I've joined the game, and has a big tendency to make a loose calls. I believe he opens pretty wide from UTG(~20%), and calls 3bet In position with most of his range.

HU Flop Qc9d7s. I cbet 65, he calls. We block AQ&KQ, have decent equity against he's calling range, so I plan to barrel a brick turn, or if we improve
.
Turn Jd, pot 271$. On such a turn I decide to give up. Villain check back.

River Th. I have 485$ behing, villain covers. As AK is very likely to be in our range, I don't want to make a massive overbet here and give villain a chance to hero fold K, so I'd rather go for a 275-300 bet to extract max value from a K, or for 150 bet, trying to get called from 8x combos, and 2pair combos.
What do you think about optimal sizing here, against described player?

I'd bet a little more on flop.

As played, bet $120 - $150. He might call w/ two pair and I'm sure will call with a K.

I think an over-bet will only work if he has a K, so might be worth it.

BTW, how does V see you -- this makes a difference, too.

I don't think going 1/2 to 2/3 pot is bad, but only getting called by Kx regardless, so bet whatever you think he'll call. I would bet pot or a little over, like 275-300. Doubt he's good enough to fold a K when he blocks AK.

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This is player dependent. All options can be good.

With my image I would bet $200 and get called by wide range vs most. May jam Vs this guy