I think it would be a great idea if like perm-a-petting your kitty you could buy with Lindens or use KC dollars or gems to get a position to change the size of a kitty.

Mostly I tend to think that it's a fun thing to keep the random sizes as they are - a happy surprise that we can't do anything about - but when you mention jewels, I think yes, I agree. This would be a worthy thing to do with jewels. Great idea.
Menagerie dollars... this would be a good menagerie incentive.
i think a different version of this might be fun: you can alter the size of your kitty, but only AFTER permapetting it...not sure if i'd do it or not...but it might be interesting.
I really think sizes should be breedable. That would give us another trait to work with, plus then breeders can specialize in size. Also, I really need to see a wild tiger sized kitty. One I can walk around with on a leash. OMG I so want one!
Making size breedable would destroy the current market value for non-Normal, especially Mega and Tea Cup. There would be a mad rush to the extremes. They'd no longer be rare (difficult to find find) and, most likely, would quickly become more common (that is, found everywhere) than Normal.
I like the fact size is a surprise so frankly I like how it currently is.
oh yeah leave sleeping kitties lie and leave the sizes alone, that would destroy the current market