The luggage has to be able to fit either under the seat in front of you or in the overhead compartment and they are all standard in size requirements. Yes, you can take one carry-on luggage and a personal item, such as a backpack. My backpack has my Ipod, Passporter, camera, wallet (I don't bring my whole pocketbook & I take everything out of my wallet that I will not need during the trip - less to loose), camcorder, etc.
Make sure that ALL of your toiletries are less than 3 oz. Go to Walmart or Target and you will find a ton of travel items, including laundry soap, that are within the travel guidelines. If you prefer larger versions, you can always get a set-priced Priority Mail box from the Post Office and place the items in that, then just mail it down to your resort a few days prior to your arrival date. Make sure you put your name & check-in date. Remember that all of the Disney World resorts have soap & shampoo in the room and will replace them each day.
I found that rolling my clothes saved me alot of room. Also, I only bring enough clothes for 5 days (of a 10-12 day trip), then do laundry mid-trip. I'll bring 5 shirts, 2 pairs of shorts, jeans & a sweatshirt (which I will usually wear, as I go in November/December), bathing suit, 1 pair of Crocs, undies & new socks.
Airlines' policies regarding carry-on luggage are NOT uniform. Of the airlines I quickly checked, AirTran - cannot exceed overall dimensions of 55" (length+width+height). SW - limits dimensions to 10"X16"X24", and AA - should not exceed 45 linear inches (length+width+height). AA also states that they may further limit carryon baggage depending on cabin storage capacity.
I strongly suggest you call or check your airline's website prior to flight to confirm carryon limitations.
I thought all of the airlines had the same dimensions, but I see that I was wrong.
