When your opponent has a perceived weak range like middling pairs, and we are going for value, in general how do you come to a decision about the size to use? do you lean towards 1/3 - 1/2 to keep them in? or charge them more (75%)? or larger? is it ok to change this sizing when bluffing vs value? eg betting larger when we don't have it and smaller when we do have it?

Assuming your opponent isn't a rocket scientist, in general I would bet small for value to make sure I get paid off by weak hands versus betting just large enough to make it too painful for them call bluffs (assuming they have a fold button, I'm telling a reasonable story, etc.).

Solid expert players might be able to see thru this method. But if you're playing against too many of these guys then you're probably sitting at the wrong table.

GcluelessNLnoobG

Are we talking about flop bet sizing, or turn, or river?

Depends on the flop texture. Weak pairs are less likely to call when there are higher cards on board, but more likely to call when there are potential draws on board.

The higher the cards on board, and the more high cards there are, the less we can bet to get value from weaker holdings. The lower the cards on board, and the more draws, the more we can bet to get value from those holdings.

To GG's point above, if you're betting big when you're bluffing and betting small when you're value betting, decent players may sniff that out and start exploiting you. Likewise, it can be dangerous to bet smaller on boards that favor our perceived range, and bigger on boards that are generally better for our opponents, because they can raise for value or as a bluff.

As a general rule, I try to have a default betting size for most board textures, and only go bigger or smaller when there's a combination of specific variables that warrant it. One of my favorite lines is betting less than 1/3 pot on flop, and betting over 2/3 on brick turns.

It's hard for weaker opponents with medium strength hands to know how to properly defend when we bet flop small and bet turn big.