Ax suited is limpable for the min, and preferably with as many people in the pot as possible. You should limp in with it when you feel there won't be a raise back to you. For that reason it's best done from the blinds or very late position (button, one off the button). Only at a very passive table can you afford to try limping with this hand from middle or early position. If a raise of 3xBB or more comes down, you pretty much have to fold, and you're leaving yourself wide open for this by trying to limp from early position.

As far as table size, it's a good limping hand with more players - full ring games are ideal. Not such a great hand at a smaller table like 6 max, where pre-flop aggression may be higher, pot odds are consistently lower, and people will tend to play a wider array of hands that usually beat yours (i.e. A10 or A9 vs. your A2/A3/A4). In really short-handed play (heads up or 3 players) it's OK just because you have an ace - if you hit top pair you're probably on the best hand, even with the bad kicker.