Every time it is your turn to act at the poker table, you have choices to make. Before the flop, if there is no action before you, you have the choice to call the big blind or bet. After the flop, if there is no action before you, you can check or bet. If there is already action, you can raise, call or fold. Considering the optimal bet sizing in each of these scenarios where you choose to bet is essential to your long term profitability in the game. By choosing the correct bet size, you are trying to maximize your wins and minimize your losses. You are trying to manipulate your opponent to your advantage. You are trying to outmaneuver them by using the math and psychology of the game.
The first consideration is that any bet should always be in relation to the current size of the pot, not necessarily your stack size. Keeping in mind how much is currently in the pot is key in determining what pot odds you are giving your opponent by your bet. Each time your opponent makes a call with incorrect odds, you are making a theoretical profit off of their mistake. Thus it is important to make good size bets in relation to your hand strength. While an opponent may make a bad call and still win the hand by getting lucky, they still made a long term mistake in your favor.
The dynamics of bet sizing differ from the different forms of poker. For instance, you may choose different bet sizing in cash games versus tournaments. In cash games, it is common to raise preflop three times the big blind (+1 for any limper), and continuation bet on the flop or turn around 2/3 to 4/5 of the pot size. While in tournaments, at different stages, players will only raise 2.2 to 2.5 times the big blind and continuation bet less than half the size of the pot. This different bet sizing reflects the different dynamics of a cash game where you can rebuy at any time and you want to maximize your potential in any given hand versus a tournament where the chips are your life and you want to minimize the risk of any one hand taking all your chips.
One important consideration in determining your bet sizing is are you giving away information to your opponent. If you bet big when you have a strong hand and bet small when you have a weak hand, then you become too predictable to your opponent who will eventually exploit that pattern. That is why many players will use a consistent bet sizing regardless if they are strong or weak, thus giving away no information to their opponent.
If you are going to bet or raise, you want it to accomplish something positive. That can be by giving you information as to what your opponent has, or getting them to make a mistake. One big mistake that many inexperienced players make is to minimum bet or minimum raise. Raising or betting so little accomplishes very little and can cost you considerably. You are giving your opponents good odds to see a flop with any two cards, or to chase their weaker hand to eventually beat yours. You don't gain any real information and inflate the size of the pot.
Let's take one example of bet sizing in a $1-2 cash game. Your are dealt A,K preflop. You only raise twice the big blind to $4 in early position. Your opponent only has to call $4 to see a $7 pot, or $3 in the small blind, or $2 in the big blind. They are getting good odds to call with a wide range of hands that could outflop you. If you raise to $6 or more, you are decreasing the odds and making it more costly for them to play their weaker hands. Let's say there are two callers. On a flop of k, 10, 8 with two spades, you have top pair with the top kicker. If you choose to bet small with your strong hand, you are likely to have all kinds of hands call (J,9 or Q,J or any two spades etc.) So while you are likely to have the best hand at that point, if you don't bet a large enough amount, the other players will try to hit their draws to beat you. With the open ended straight draw, they have 8 outs or roughly 16% on the next card, or 32% by the river to improve to a better hand than you. With a flush draw, they have 9 outs to improve, which is 18% on the turn and 36% by the river. If you bet 1/2 or less of the pot on that flop, you are giving them correct odds to chase their draws. If you bet 75% of the pot, they are chasing against the odds, and thus making a long term mistake, even if they hit their draw in the short run.
Just as you are gaining information from how opponents react to your bet size, they are also picking up information as well. Sometimes you have completely missed the flop and you are bluffing trying to represent strength. This is one reason that many players use a consistent betting size to mask when they do and don't have a strong hand. On the other hand, your opponents also know that you can't possibly hit every flop, so you need factor that in as well. One piece of advice in NLHE would be to avoid ever hitting the bet-pot button, just because it's convenient. You are likely to scare off your opponent if you have the best hand and likely to waste a lot of chips if you don't.
Your goal is to make every play and bet for a reason. Are you trying to get your opponent to fold a better hand, or to value bet? With a continuation bet, you want to make a bet that will accomplish your goal while saving you money when they have you beat. If your opponent is intent on calling you regardless, why not bet a slightly smaller amount to save money when you are beat. Reading your opponents, their tendencies and reactions is key to helping determine the optimal bet sizing. It is a constant dance that you would benefit greatly from focusing on closely.