The bubble on my 8099 is pretty good size - maybe between 1/8" and 3/16". The bubble is not impeding the rotation of the disk at all and it is still taking bearings that match those of the other 8099 without the bubble (and matched my Suunto M3 Leader within a degree or two).

I don't think the disk actually floats. The Brunton FAQ says that the 8099's (as well as the 8097, 8096, 7DNL, and 15TDCL) have a sapphire jewel bearing, so I think it actually pivots on the bearing like the needle-based compasses. Its just that it is a magnetized disk rather than a magnetized needle. I can see that the higher surface area of the disk could risk higher adhesion to the bubble's surface tension though.