JetBlue boarding groups are as follows. First wheelchairs. Second those with "disability". Third Mosaic members (high level frequent flyers). Fourth "Even More" seats (boarding group A). Fifth- rear of plane (group B). Sixth middle and then finally front. So sitting in the rear actually puts you in a higher likelihood of fitting your carry on. On the E190s the carry-ons have to go in sideways rather than wheels first so there is much less room for carry-ons overall. Unfortunately, flights out of FLL always have a ton of wheelchairs and "disabled". Still, probably not enough to impact your space at the rear.