Untill now, I thought the general rule was to cbet big on wet boards / small on dry boards.
After a quick search I found an article from upswing saying exactly that : https://upswingpoker.com/c-bet-sizin...tinuation-bet/
It was published 5 years ago.

But in a recent article on monotone flops, also from upswing, they say :
"Any time that you are c-betting and the board can change drastically on the next street, you’re usually going to favor smaller bet sizes."

I know there are other factors to consider when chosing a cbet sizing, but I'm confused about what to do generally on wet boards.

There's no mathematically rigorous definition of "wet" or "dry" that I'm aware of. Not much meaning is derived from their usage although their usage persists for some reason.

The question itself if flawed for that very reason.

Took me literally 5 seconds to think of a toy game where this is not true. What is "smaller" anyway? That's not a precise statement at all. Suppose IP opens 2bb with {55+, A4s+, ATo+, suited broadways, KJo+, 9Xs, T8s, K7s+} and BB defends relatively close to Nash equilibrium. Flop is 9s6s2s. Is half pot "smaller"? I'd say no; that's "moderate" sizing. Yet that's going to be the most frequent unexploitable sizing on that board.

Consider a toy game where:

1. IP has only AA and 3d2d, 3c2c, 3h2h t
2. OOP has 88-KK; all spade combos; all 9Xo; all 8Xo (but not 98)
3. Flop is 7s6s5h; SPR is 10. (Pretty "wet", right?)

As I understand it, so as always take that with a grain of salt...

In the most abstract sense, my understanding is that equities tend to run close together on wet boards between the PFR's range and hands that will continue. Therefore you don't really want to start building a big pot until you get a favorable turn, which puts more pressure on villain as they have only one card to come instead of two.

For a specific example, take a flop like QT9 with a flush draw. Lots of hands will hit this board in a way they're going to continue on the flop whether you size big or small. Sizing too big at best will fold out the weakest parts of villain's range and at worst put some of your good-but-not-nutted hands in a rough spot if you get check-raised and continue to face aggression. Do you really want to blast off with, say, AA on this board? Or is it better to bet 1/3 pot or so and then hit the turn with a big bet on a brick card?

So the theory there, I think, is that you want to control the pot size until the point where the range equities start to diverge significantly. (I suppose if you flop something nutted on a board like this-- like you have QQ/TT/KJ-- and you don't think villain is going to be paying much attention to your bet sizing, you can bet bigger.)

A corollary is, these can be good boards to triple-barrel bluff with hands like small pairs, because villain will have more hands that have the equity to continue for two streets but can't call a third (and in fact will often be more inclined to fold because they'll have blockers to your natural bluffs, whereas your hand unblocks all the pair+draw / combo-draw hands that can't fold the flop or turn). Something like JTo, A9 with the flush draw, KTs with the backdoor flush draw that comes in on the turn, is going to be hard pressed to call three streets if they haven't improved their made hand by the river. And while I'm less sure of this, it's possible that even if one of the draws comes in you can still do this profitably to bluff them off the other one (or at least try on the turn).

I'm not entirely confident in all of that, and it's pretty late as I'm writing this, but I think that's the concept here and how to put it into practice.

This looks like a better rule of thumb than my previous "wet flop = big bets".

I need to work on these spots with like QT9 flush draw.
I play low stakes, value oriented against many sticky villains, so I usually want to put chips in with AA even on the worst possible flop.
But there is room for improvement there.

A wet flop is one where one or more players will have high draw equity on average. I'm sure you could make something mathematically rigorous out of that.

The way I have heard it described is that kind of wet flop - factor generally increases bet size. Extremely wet flop - bet size goes back down.

For example, UTG vs BB 100bb, single raised pot, J95 is going have more large bets, 50%-75% pot.

Compared toJ109 which is going to be reflected with more 33%-50% pot sized bets.

On the second board, there is the possibility of flopped straights, and a lot of turns put a one liner our there. Those types of boards are the really wet ones where sizing goes back down.

Think about a BTN vs. BB situation where the board comes 8h6d3c vs. KdTd4h.

However, on 863 rainbow, the BTN bets only about half the time but often with a huge (overpot) size. Yet on KT4 two-tone, the BTN basically range bets for maybe 20-33% pot.

Why is that?

On the 863 rainbow board, the BTN's range advantage isn't nearly as big, but their nut advantage is enormous. This is because while the BTN has all of the overpairs and sets, the BB does have a lot of 8x and 6x hands. This incentivizes the BTN to bet big with the strongest hands in their range (like overpairs and good top pairs) to stack the BTN's less-good hands. On low card boards there's also a general incentive to bet bigger in a lot of cases because those are the boards where the best hand will most often change.

On the KT4 two-tone board, the BTN has a much bigger range advantage and a nut advantage. The BB's range also contains way more trashy hands. The combination of those factors makes the BTN more inclined to bet small to allow their entire range (including their trash) to realize its equity on the flop. On the turn, once the BB has folded its absolute trash, ranges will narrow and the BTN will do much more polar sizing, betting its strongest hands and draws (as well as its worst hands) for a big size (like pot).

So your sizing is primarily determined mostly how your range performs against your opponent. As your opponent's range gets stronger (either because they're stronger preflop or they folded hands in their range to bets) you want to bet less often, and generally with a larger size. As equity completes (i.e., the nuts are on the board like with a straight or flush draw) you will tend to bet smaller than when there are still draws available. That's why you'll see on boards where there's already the effective nuts available on the flop (like for example, a monotone or paired board) you tend to bet smaller, even if your range overall has a significant range or nut advantage.