what size mdf for a sub ring

For a CompVR 12, 3/4" is fine.

I always like to double it up, though... if possible, do it.

what do you mean by double it up?  this is what im looking to do.  http://volvospeed.com/Mods/speaker_rings.htm

if i do it like that with the 3/4 mu sub is going to sink down into the enclosure about 1/4"  the outside of my sub is only 1/2" in depth.  wouldnt that look kind of funny?  rather than doing like 1/2" and having it almost perfectly flush with the fiberglass enclosure?

Oh, I thought you were talking about the actual baffle board that the sub mounts to.

For this case, I think 1/2" would be sufficient. It depends on how far the surround is from the baffle -how far it sticks out from the baffle. Measure this, then use that thickness of wood.

It's the actual board that the sub bolts to -the front of the box, on regular square boxes.