At the RD by me, they have the small "3.5 and down's, which come 3 slabs to a cryo pack, and the mediums, which I believe are 3.5 lbs and up per slab.
To borrow from Craigs site that Larry linked...
3 1/2 & down, 4 & over, etc. This is butcher talk for the weight of a slab of spareribs. A "3 1/2 & down" weighs 3.5 pounds or less untrimmed, with the tips attached. Most chefs prefer theirs 3 1/2 and down, from younger hogs.
I bought the medium ones for our last comp, and regreted the decision. They are only 2 to a pack, and they're quite a bit larger than the 3 and downs, and both packs I bought needed to be extensively trimmed due to the excess of meat and fat on them. I'll stick with the 3.5 and downs from now on!
I'm not aware of any weight restrictions on "baby backs" but backribs weight usually depends on how much loin meat is left on when they're butchered