One further note on this, after the flop, if a player goes all-in, but can't make a full bet, i.e. the size of the big blind, the next player only has to match his all-in bet, not the big blind/full size bet.
Here is a note on all-in before the flop.
What happens when a player can't cover a blind? (All-in on a Blind)
If a player in the big blind position does not have enough chips to post a full big blind, he must go all-in with all his remaining chips. Even though this all-in bet is less than the full big blind, the 'bring-in' (minimum bet) is not lowered for the other players. If the blinds are 100-200 and the big blind goes all-in with 125 chips - the other players must still post 200 to remain in the pot. The 'bring-in' remains at 200 and the small blind must post 100 additional chips to call (assuming no raises). The big blind player all-in for 125 may only win 125 from each other player in the hand, he cannot win the full 200 big blind from each player.
An example: Assume the blinds are 200-400. The small blind posts 200 but the big blind posts only 150 (all-in) instead of 400. The remaining players must call the normal big blind bet of 400 to remain in the pot. The bring-in (opening bet) is not lowered to 150. The small blind must add an additional 200 to the pot to stay in the hand (assuming no raises). The big blind (all-in) player may only win as many chips as he bet (150) from each opposing player. A twist to this example is that if one player calls the 400 big blind and then the small blind folds, the player who called is refunded 300 from the pot instead of 250. He receives 250 to make his bet equal to the big blind all-in bet of 150 and he also receives 50 of the 200 bet that the small blind posted. The pot is now 450 (150 x 3 players).