I always raise potsize (3.5bb) pre-flop, +1bb per limper. People usually raise 3bb to keep the pot slightly smaller and leave more room for post-flop play. Like on NL100, if you raise to $3 and button calls, the pot is $7.50, and a 2/3 pot cbet would be $5. So if button calls again, the pot on the turn would be $17,50.
Now, if you raise to $4 in the same scenario, the pot on the flop would be $9.50, and a flop cbet would cost you $6.50, so if you get a call the pot would be $22.50 which is only a difference of 5bb, but it really shows in the long term. You can imagine the difference on the river.
On lower stakes you mostly play for value, so you'll opt to extract as much value as possible, therefore it's best to raise 4bb at NL10.