Hey there,
this is a feature request (can I set that, or can only you...?). I would ask for something like
SLIME@1 SLIME@2 SLIME@3 ... SLIME@n
where the number represents the slime size. And yes, that's definable however you like it (I spawned a size 30 slime with CommandBook once). I want to alter how bigger slimes spawn smaller slimes a bit, so that would be great :)

I spawned a SLIME@128 in testing (it's a bit buggy though :D). I thought that defining drops for SLIME@n would already work - have you tested it?

Err... It works. Sorry for bothering you, but it was not in the list here:
https://github.com/cyklo/Bukkit-OtherBlocks/wiki/Material-and-Creature-Values

You could add it there, it works for me. Slimes@4 now drop 2 @3, @3 drop 2 @2, @2 drop 2 @1. Happy 8 Slimes from just one @4! Big fun! xD
Thank you for your plugin anyway (:

I made allowances for numerical data on slimes, so it should work.

Cool - glad it worked, I've added details to the wiki pages.