Harrington seems to recommend 3-4.5xBB and vary it up, so they can't read what is in your hand. For example, if you always raise AA 4.5xBB, they will always know when you lead out with it...

My thought is the same as everyone else... except at micro stakes... 3xBB won't even get noticed as a raise in micro stakes... I've found that you have to go 5xBB to 7xBB at the .01/.02 level to even get noticed as raising.