PokerStars - $0.25 NL FAST (6 max) - Holdem - 6 players
Hand converted by PokerTracker 4

BTN: 100 BB
SB: 120.84 BB
BB: 297.04 BB
Hero (UTG): 100.4 BB
MP: 79.6 BB
CO: 100 BB

SB posts SB 0.4 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.4 BB) Hero has 6 5

Hero raises to 2.2 BB, fold, fold, fold, SB raises to 10 BB, fold, Hero calls 7.8 BB

Flop: (21 BB, 2 players) 8 A 7
SB checks, Hero checks

Turn: (21 BB, 2 players) K
SB bets 10 BB, Hero calls 10 BB

River: (41 BB, 2 players) K
SB checks, Hero bets 12.92 BB, fold

Hero wins 39.16 BB

What do you think of this small size river bluff? Villain will probably only call with an A/K. Therefore I could go big or small, but I think small will accomplish as many folds.

You could start the bluff sooner in the hand. On the flop, you can definitely bet, you won't have any better bluff candidates. Oesd+bdfd+no showdown value+potential 4th pair.

You turn a FD to go with your oesd and still no showdown value. Not sure if you have a raise range on such a turn, but if you do, this will be one of your better bluff candidates, with high equity and no showdown. However, there's also a good argument for just calling, as you realise your equity and avoid the possibility of getting jammed on, which isn't great for your hand.

Once you get to the river, even though you have two s which you really want villain to have, the 5 and 6 are far from the most likely spades he will have if he has a FD. As your hand has no showdown, I feel like it is a mandatory bluff. You can have some Ax and a lot of Ks which will want to get value. Your raise size will probably get called by most (all) 99-QQ and some 7x-8x, and I would say you are repping a small A, maybe the highest pp you can have (JJ/TT?). So you will get a fold out of a lot of better hands, but I don't think you'll only get called by Ax and Kx. MDF for villain will be about 75%, so he should probably be calling with just about every made hand he has. I do reckon villain will be slightly overfolding here though, as he should have a decent amount of missed draws and air in his range, so this might be a spot to exploitatively over bluff.

You could also go larger, and rep a bigger A or a K. However, if you do that, you'd probably want a hand which would block some more likely Ax and Kx hands villain would 3bet pre, so maybe T9s, JTs, or QJs. Even though villain's check otr probably means a K isn't a huge part of his range.

Also, worth considering that if you bet small, you shouldn't be bluffing as much as when you go larger. With your sizing, I think ~20% is the bluffing frequency you should be aiming for (although I think here going exploitatively larger is a good decision), so worth considering how many more bluffs you'd include in this line.

bet flop. if you're checking any 65 its never the bdfd combo imo.

otr when you're playing against other regs i'd recommend just thinking about thing in the opposite way. like your logic is ok vs a fish but you're trying to play hand vs range rather than range vs range against a presumed competent player - you might end up getting exploited over time - and its certainly not training you to get better and take on better competition. like by your logic here, you shouldn't be value betting anything worse than a strong ace, basically AQ or maybe AJs the times you check the flop with them (and you'll bet them sometimes so its not that many combos in practice), because he won't call with worse. a lot of your betting range will be KQs/KJs(/KTs). so you'll probably end up betting a size where, like pjj said, you should be bluffing 20% of the time, with a bunch of bluffs (of all your no-SDV hands this is one of the worse ones because of the spades, so we have to assume you're bluffing every missed draw if you're bluffing this) and a few Ax combos. then when you have a K, you'll bet bigger, and probably have no bluffs in that range. if villain is paying attention he can essentially call the small bet with any pair (maybe even some QJs/QTs) and always fold to the big one unless he was planning to value x/r.

instead, start from your value range and work backwards from there. we've identified two types of value hands: strong Ax, and Kx. Our Kx wants to bet big, maybe even overbet, to get value from his Ax. Our Ax wants to bet comparatively smaller. So your choices here (and i think both are valid) would either: be to split your value range into a big size for Kx, like pot or slight ovebet, and maybe a 1/2 pot size or something for AQ and AJs, and distribute your bluffs to each (putting more bluffs in the big range because it needs more to balance, whereas 1/2 pot only needs 1 bluff for every 3 value combos); or just have one size that's like 2/3-3/4ish. if you want practice choosing between the two, i'd literally count up the value combos and bluff combos you have on the river, and try constructing balanced betting ranges for the split sizing scenario, and then for the one size scenario, and see which works better for the number of value and bluff combos you have.

you want to bet bigger and rep the K, nothing in your range needs to go for thin value
AQ can bet bigger for value too, rest of your aces are too thin