If you're flying out of JFK I'm guessing its usually the airbus A320 since JFK is their hub. So with the bigger planes you can carry on a 26" suitcase & personal item... my guess is your suitcase is just about that size.
We always fly jetblue and can do 10 days to DIsney or Caribbean/mexico in a carry on & back pack... our carry on is only 21" including wheels. 26" is a pretty good size to allow for carryon