[grapabo]

Standard is between 3x to 5x the BB. The reasoning has to do with the pot odds you give anyone who wants to call. If it's an unraised pot with 1.5 BB to start, raising it cold by 4xBB will make it 5.5BB, 4BB to go (less to go for the blinds, obv). These kind of pot odds will make it harder to call with a junk hand, and gives you information that anyone who calls more often than not has a hand worth playing. And at the high end, anything over 5xBB, barring a wild game, is unnecessary - you can get the same information at a lower price.

I assume the 3.5BB default is for the benefit of the multitablers so they don't have to manually measure out the raise?