Common poker knowledge states that you should be raising 3xBB +1BB for every limper. Why? Well, because someone at 2+2 said so. As a default, it's not a bad idea. It works because it gives the limpers/blinds (people you have position against) 2:1, decent odds to call your raise with worse, while giving worse odds to the players behind you who haven't put money into the pot yet. When stacks are realtively deep and you have no specific reads on the players, this is generally the best play. But there are reasons to change it up.

That guy calls everything!
If someone limps in front of you and has shown that he's never folding here, raise more with your premium hands. Against players like this, I'll keep raising an extra BB more until they fold. Then you know where their limit is, and you can raise it every time with your best hands. Think they're going to adjust and start folding? They're not. And if by some miracle of fishdom that they do adjust, you can readjust and make it a little smaller. These guys love to see flops... make them pay the maximum when you have it.

Damn these blinds.
Being on the button provides a few different reasons to change up your raise sizing, most of which suggest betting smaller than normal. If the blinds are really nitty (tight), raising smaller will make your steals cheaper and get more calls when you have good hands. Playing hands in position is a goal in and of itself, and raising smaller will help you to do it more.

If the blinds are aggressive, raising smaller allows you to lose less when they 3bet you, making their 3bets less profitable and your steal attempts more profitable. 'Nuff said. And if they're fish who call anything, follow the first point. Raise big if they're going to call alot.

Ineffective Stacks?
Especially in tournaments, sometimes stack size just isn't right for raising the standard. Situation: you're in a deepstacked tournament during the first level and you're dealt AA in middle position. Everyon folds to you. How much do you raise? I would recommend larger than normal. When you raise to a normal amount with huge stacks, you're going to get tons of calls from small pairs and suited connectors. When they call, you've got a problem: you're not getting money in the pot unless they hit their hand or you see an unlikely set over set. It's a situation where you can lose a lot or win a little. When you raise big, you'll get action from the hands that will pay off when they're second best: big pairs and AK.

When effective stacks (the size of the smallest stack in the hand) are small, raising standard sizes puts way too much in the pot and could commit you. You can get smaller stacks to fold for less money, and more calls when you have a big hand with enough money in the pot to get it all in by the river. A good player's biggest advantage comes postflop, not preflop, and the more money in the pot, the smaller that advantage will be.

Throw your antes on the stage.
In the later stages of a tournament with big blinds and antes, stealing becomes more important than anything else. If you don't do it enough, your stack is crippled. If you do it too much, you're out. Betsizing can help.

If your stack is small (M of 5 or less, or about 10bb, your only real options are push and fold. The reason is that, if you raise, you're committed. So you might as well use your whole stack as leverage and win the blinds and antes as often as possible. If your stack gets much smaller, you lose your fold equity, which is pretty much the lifeblood of a tournament player, so you want to be stealing a little more often. But that's a topic for later.

If you have a comfortable stack, raising big is often a mistake. If there are no limpers, 2-2.5x performs the same function. Chips are valuable in the ante stages, and good players will want to use them to maximize fold equity, not call and miss the flop 65% of the time. Because everyone is stealing more and there's fold equity to be had, lots of re-stealing (3betting) happens in these stages (though not enough at small stakes). When you raise smaller, you minimize your losses when this happens, saving those oh-so-valuable chips.

Getting some definition.
Raise size can be messed around with based on stack sizes, player considerations, and game situations. It should also be heavily affected by SPR, but that's a complicated topic better explained by Ed Miller. Don't fall into the trap of raising the same amount no matter what. Be creative, but have a reason when you pick a size.