If you check the JetBlue site they give you the maximum dimensions allowed per bag. Each ticketed passenger is allowed one checked bag free, one carry and one personal item. The checked bag can be no larger than 62" (that's length + height + width). Those measurements include all handles, wheels, bulges, etc, so be sure to include those in your measurments. When you go to buy luggage carry a tape measure. The manufacturer's listed measurments do not include wheels and handles, so most bags are actually a little larger than they are listed.
According to Jet Blue's web site each person's personal item can be no larger than 18"x15"x8" and must fit underneath the seat in front of you. The maximum size of the carry on depends on which type of plane you are on. The Airbus A320 allows a carry on of 26"x18"x12" and the EMBRAER 190 allows 24"x16"x10". Again this in including all wheels, handles, and buldges. I'd go with the smaller bag regardless of which plane you are on this trip, as your next trip may require the smaller bag.
As far as what kind of bag to buy, that's up to your personal preference. Some prefer softsided suitcases, some prefer hard sided suitcases, and some like myself prefer rolling duffle bags. The main thing is to get the lightest weight bags you can. You can find cheap bags at places like Target, TJMaxx, Burlingtons, or Marshalls, but while some are lightweight and work great, others are too heavy. The lighter the empty bag is, the more you can pack into it without going overweight. I like the large rolling duffel from LLBeans for a checked bag (the extra large is too big for a checked bag though). I have a large one with my last name monogrammed on it, that's bright lime green so it's easy to see at baggage claim, it's very lightweight, and it's lasted for years. I also have some Heys solid spinners that are very lightweight and easily maneuverable. I'd try to get something different from the typical black or navy that everyone else uses to make it easy to see coming through baggage claim. Otherwise do something to make your bags stand out (colorful duct tape around the handles, bright ribbon or bandana on the handles, etc.) Also I'd never buy bags without wheels (either the two wheels or the four wheel spinners). Wheels make it so much easier to get through the airports.
Also take into consideration that if you have kids to get through the airport, can those kids handle two carryons each or will you be wrangling the kids carryons, plus yours, plus the kids?
As far as weight, just weigh them before leaving home. Any suitcase can get over 50lbs depending on what you put in it. I use packing folders and packing cubes, which help me organize my bags and allows me to get more in there, but I have to watch the weight as I can easily go over 50lbs when I use them. Alot of people pack in space saver bags or zip lock bags and again have to be careful not to go over the weight limit, because they allow you to get more into each bag.
Oh yeah, one last thing: divide everyone's belongings among each of the four suitcases, that way if one bag gets lost that one person isn't stuck without something to wear.
