2mm stones are "3 pointers" (as in .03 cts) and I think that's generally how melee diamonds are described, at least on PS?
From a quick search over PS, it looks like .5-1 pointers will give you you a very thin halo, and 1.5-2 pointers a slightly more substantial one, though still delicate. I didn't find any 3-pointer halos in my quick search.
Here's a ring with 3 pointers in the band: http://www.briangavindiamonds.com/engage...te-gold-5483w18
It seems really pretty in the band, but I think it might be a bit too big for a halo, unless you're setting a HUGE stone or want a chunkier look.
From an online site, here's a conversion table for points to mm for round brilliants:
0.01ct (1 point) = 1.3mmm
0.02ct (2 points) = 1.7mmm
0.03ct (3 points) = 2.0mmm
0.06ct (6 points) = 2.5mmm
0.10ct (10 points) = 3.0mmm
0.25ct (25 points) = 4.0mmm
0.50ct (50 points) = 5.0mm
0.75ct (75 points) = 6.0mm
1.00ct (100 points) = 6.5mm