MTV's South Park Avatar Creator is 9.3MB. Three other avatar creators that seemed similar in the appstore averaged about 6.5MB. Not a very scientific comparison, admittedly.
I wouldn't be surprised that they're bigger as it's generated code. Still, it doesn't seem to me like flash apps are going to eat up all your available app storage space.
Tweetie 2 (very full featured twitter client): 2.8MB
Things (todo list manager): 983KB
Strategery (game, simple graphics): 6.3MB
Canabalt (game, slightly fancier graphics): 8.8MB
iOctocat (github browser): 749KB
Prowl (internet notification client): 351KB
Simplenote (Note taking, iPhone/iPad universal version): 853KB
Edit:
A cursory run-through of the flash apps listed by Adobe http://labs.adobe.com/technologies/flashcs5/appsfor_iphone/ indicates that they're all approximately 10-11MB.
MTV's South Park Avatar Creator is 9.3MB. Three other avatar creators that seemed similar in the appstore averaged about 6.5MB. Not a very scientific comparison, admittedly.