I am no expert but between the 3 pairs of monitors I have had (all 5-6.5 in woofers) and being in other rooms with similar, or bigger woofers, I would say go on the small side. I wish my BM5a's had more bass, sure, but they are pretty good other wise. I use a sub to reference my bass, not when mixing.
There are a few easy tricks to get a good idea about bass for mixing as well. Bass builds up so move around the room. I know in one corner of my room, if the bass sounds right there, it will sound right. I also know if I turn my LF eq on my mixer to a certain level, the bass will sound right. I use those two as a quick reference and it's right most of the time, takes about 3 seconds to check.