Any good guidelines here?

A friend had a spot with SPR ~6, and a flop of A73 monotone. He was preflop raiser (from say cutoff) and was heads up vs BB.

I'm assuming the BB checked to you after defending. With the monotone board I'm betting between 1/3 and 1/2 pot. IMO this is the best bet size for a dry king high one card flush bluff, a flush by itself (including the nuts) and if you actually have a good non flush hand such as a set or top two. If I completely miss I'd still put in about this size bet on a monotone board but how often I'd do it would depend on my opponent (e.g. is he sticky, tricky, tenacious) and my image relative to him. Sometimes even in position head up you need to just give up especially if the board hits your opponents defending range and he likes to check raise.

Be warned that I play in very loose live games sitting down in a casino with living players I can see so the last time I was head up with the BB was a year ago when I was helping start a game. Because of this my eyes glaze over reading hand histories from 6 max online players who play games that often get head up. Those situations rarely apply in my games.

part of the problem with discussing and learning bet size strat is the range of choices is wider than the more discrete 'bet / check / call etc'; the logic can be very clear and obvious for why we bet rather than check, but it's difficult to apply that same logic here. what that means in part is that i've generally refrained from giving bet-sizing advice - my own bet-sizing strat has changed a lot over the last year or so. you could post a hand and one person says 80% and the other says 20% and an argument ensues - but they could both be right. Which implies that 50% could be right too, both under poker theory and solver strat. It depends on not just your opponents but your own style of play, too. Bet-sizing advice is always going to be difficult for that very reason.

Yes, I agree. Borrowing from my NLHE experience, I'd think this is a board we can bet range for 1/3 or even 1/4 pot. It is a very dry and static board. We have a range advantage.

My friend bet much bigger; I suggested we can start with small on flop, then polarize turn/river with appropriate hands to get the money in, or take a free card with hands that are medium strength.

just know your pot odds and outs and number of players in a hand. nothing else matters. its just math.

Huh? Bet sizing here doesn’t matter?

brudddah, look at Fred title?

This post has me more confused than the last one!

EV difference isn't big between bet sizes in game unless you're talking extremes.

Rough guidline I'd recommend:

SRP
50% on unpaired flops
33% on mono flops
33% on paired flops
50% on straight flops

someone find me an ermergency mod asap