It's a straight power law conversion, 2^class = tonnage
Class 1 Racks = 2T
Class 2 Racks = 4T
Class 3, 8T
Class 4, 16T (this is the largest rack the Cobra will take)
Class 5, 32T (Type 6 & Asp)
Class 6, 64T (Type 9 & Anaconda)
Doesn't seem to be any quality changes on the racks, they all seem to be grade H, I haven't seen any higher grade racks, but it would be nice to see higher grade racks letting you store more cargo in the same space.
Class 5 racks I've definitely seen at Ejeta Station in 14 Herculis.