did you read through what i left you here?...
http://www.aquariacentral.com/forums/showthread.php?t=215277
it is REAL HARD to find a proper care sheet on corn snakes because they're the most common snake kept and everyone and their mother who owns one thinks they're experts.
corn snakes may be slender but the thinking that they are small snakes is way off. these guys can get over 6' long.
for tank size you should be looking for a minimum of 2/3 body length by 1/3 body length. if he ever reaches his potential that means a 2' x 4' footprint tank MINIMUM.
a 20 gallon tank is obviously too small for an adult... i'd say a 30 also. a 30 or 40 gallon snake breeder should do for quite some time (18" x 36" footprint) and if you luck out it won't get any longer than 4.5 feet and it can stay in there.
i'd say the head portions are not the only reason to feed the right size feeders. it is a serious one because if their head is too small as an adult they will not be able to take in proper nutrients and health will fade to death by malnourishment. that being said head growth is usually stunted by something other than feeders with exception to power feeding. with power feeding it's possible for the body to grow too fast for the head to feed it properly.
i have kept snakes all my life from w/c and released back to w/c and rehabbed back to health to c/b and part of the family (my redtail). that's going on 27 years now... please take my advice serious. i would like to help you provide a happy long lived home for your little guy.
it will be long lived too. up to 30 years. 30 is VERY rare but over 20 should be expected and they never stop growing. it just slows down after adolescence.
EDIT: maybe kyryah, vicious_fish or chris anderson will chime in here. imo you'll get nothing but impeccable advice from one of those three. i don't know what v/f and chris keep as far as snakes or if they do but i trust if they were to post that you could take their advice. i do know kyryah has a couple corns and she's very knowledgeable.
no... stunting will not happen because of the cage as far as i know.