I'd go bigger pre to 4x if I was Isoing but I'm probably just checking there a fair amount. As played I have no issues with the line if you have a read he can float the river with absolute trash. I'm guessing having this guy on your table was super awesome. Limp call KJo, calling flop with nothing, calling turn with GS and floating river with nothing. As long as you hit a pair its valuetown!

I think sizing is good as it is.

4x is standard but there's arguments for and against smaller sizings.

Flop and turn are good sizing

River should be a value bet imo. No need to be massive but $1.90 against a limper should get value from his Ax range

Preflop I agree that I think we should be going a little larger, 4-5x seems good to me. On that flop I don't mind betting or checking. A problem we have with betting here is that we are just going to get a huge amount of folds in general, I would assume the limpers range is fairly weighted towards a lot of pocket pair and SC type hands which will be folding quite frequently so I don't hate checking here at all, if villain does have an ace then he will be betting for you anyway.

When we get called on the flop when we play it this way we definitely want to be betting the turn as well since villain will normally have a hand that can call at least two streets after the flop call. I think I would make the turn bet a little larger with this in mind, probably closer to 75% of pot, around $1.20.

On the river I would be betting again myself here. Obviously given the hand the villain has shown up in this exact scenario then checking is better but after we have gotten two streets of action I think extremely often we are going to be against a middling ace that will be checking back when checked to so betting the river for value seems superior than the check call.

Not much I can add, but I would definitely bet bigger on the turn, it looks like our buddy has something they like and may well call again.