I can't really help either, I've never seen a "miniature cocker", I've seen some slightly smaller than the breed average and I've seen ones taller too. Even had a litter in which one of my pups was noticably much smaller than the rest but in reality his birth weight was within the normal limits for a cocker, he grew to the same size as the rest, by 8 weeks he was just a bit smaller and lighter, but he caught up and he is as healthy as the rest.
I don't really know what to say on the one hand you've picked this one out and they say it is healthy but on the other the height issue may put certain restrictions on any insurance you take out as it is unusually small. Are you willing to accept a pup with health issues and could you pay for them if the insurance won't cover this kind of problem? Maybe the pup will grow to the correct height eventually, or has the breeder bred these 2 dogs before and similar happened in the last litter and thats why they say it wont grow past chihuhua size?
I'm sure someone with more experience will be able to offer more advice.