Pre-flop: Raise size depends on position and hand strength. Generally, 2.5x to 4x the big blind.
Flop: If you have a strong hand or the board is dangerous, bet between 50% and 100% of the pot. If the flop is dry, bet smaller to control.
Turn: If you have a strong hand or a full draw, increase the bet to protect your hand. Between 70% and 100% of the pot.
River: If you have a strong hand, make a value bet around 75%-100% of the pot. If you're
bluffing, a larger bet can also be useful.
Basically, bet size depends on your hand, board texture, position, opponents' tendencies, and whether you want to protect or maximize your value.