We fly Jet Blue exclusively, my boys love the TV sets and I love a peaceful flight!

They allow each passenger ONE PIECE of luggage. For carry on, I've brought 2 WDW tote bags, a small cooler tote and my purse for the entire family, so they've never questioned our number of carry ons. This trip I'm trying to look less like a bag lady and I bought a 19 inch rolling carry on at
Walmart Saturday night for $10!! It was the only carry on I could find that was not 21 inches (the standard). Jet Blue requests all carry on luggage to be 17 inches which is impossible to find!! They don't measure at check in. I've never seen that little suitcase frame you're supposed to slide your carry on in for measurement at Jet Blue (I've seen it always at American) but you never know.... I'm only stuffing ours with swimsuits, so if they don't like that it's 2 inches too large, I'll ditch it - it was only $10. But I really don't expect them to measure it.
All the details are on their website. I know they limit carry on to one per person not including a handbag and luggage has to be 17 inches.