Well, I don't use any abnormal sized luggage. I use everything that is regulation and won't cost me extra with the airline. I put me and the two little ones 6 and 4 in the biggest. I roll the clothes. I think rolling really helps free up space, roll tight!!! I use every section possible in the bags, all zippers. It is amazing how much you can get in a suitcase and still be under 50 pounds. All the stuff I need for WDW is in the zipper sections, pockets of the suitcase, including a collapsable cooler bag, all the stuff for my day bag at WDW and the day bag, not taking it on the plane as I needed the bigger bag for all the stuff I want handchecked.
I tried using the plastic bags, but it was frustrating me by the noise. I got irritated by the noise and thought oh yeah, this will sound so great in the morning, don't want to make older siblings 20 and 12 grumpy when 6 and 4 get up at the crack of dawn..lol.
I take capris for me, shorts for the kids, NO LEVI PANTS!! I put all their things together, underwear, socks, hair thing and outfit and roll up and squeeze it all in. I can get 8 outfits for me on one side of the suitcase, stack the beach towels on top of my clothes ( I had bigger beach towels, but they were taking too much room, so switched them with the skinner beach towels, then stack the other two and put their shoes on the bottom. My shoes and two other pair of shoes go in the lid of the suitcase zipper part. The little ones have 8-9 outfits with at least 2 extra outfits for mishaps.
20 and 12 are sharing a regular size suitcase. Each has 7-8 outfits, tanks, shorts, capris, rolled the same way and squeeze together. The big problem we have with them is SHOES!! They love a million shoes to go, but have it down to 3 pairs..lol
I have two bottles of the downy wrinkle stuff to use as soon as we get to the hotel. I usuaslly will immediately unpack to avoid extra wrinkles.
H gets his own suitcase, smaller of them, and he doesn't like to take much at all. He will rewear his clothes...(a typical man) and just bring extra shirts.
I have a smaller bag that will hold the extra bath stuff, dryers, straightners, all the bags of toileteries (i give each person their own little travel sized crap so there is less arguing) plus we are splitting up two days after we get to florida, H has to work in Daytona, me and the kids are heading to WDW.
The other regular suitcase is the snack food which will then become the souvenior and dirty clothes bag. I do take advantage of each person gets a bag so if I have to use the last 6 one, then I can. I empty all the boxes and put in ziploc, boxes take up too much room. When we had one in diapers, I did undo the diaper bags and started stuffing the diapers in the zipper sections of the bags and just layering them ontop of all the clothes instead of keeping them in that bag.
Each kid and I have a carryon, all regulation. Not taking the laptop. I have a disney duffle that will hold all the items to be hand checked, dvd player, mp3 players, cell phones, charger( as we have a layover) tickets, park information, passes, itinerary, money, all goes in my carry on. Kids each have their backpacks from school full with their travel blanket (leaving at 6 am so hope they sleep some more) snacks, coloring stuff, pocket doodler and etch sketch, pocket game.
I hope that helps. I am an overpacker, but impressed that I'm keeping things to a minimum and under 6 suitcases..

And there is 6 of us...