I flew to Mexico last year in November and took my scuba gear with me in a large Pelican case. I bought the Pelican case because I was afraid of someone stealing my scuba gear. The case with the gear was over 90 pounds. Anything over 75 pounds gets hit with a $100.00 dollar fee per leg of the trip. So it cost me an extra $200.00 dollars in bag fees to get everything down to mexico. On the way back, I put all of our clothes in the Pelican and put my scuba gear in my carry on bags so I wouldn't have to pay the extra bag fees. On checked bags, unless it is way over sized, I don't think they care about anything other than weight. On carry-on bags, it has to fit in the overhead bin or they will make you check the bag. The width and height are the most important sizes as this will determine if it will fit in the bin. The length of the bag is less important. If the flight is booked solid, and there is not enough room for your carry-on luggage, they may make you check it but I believe they cannot charge you extra. If you have two bags, put the smaller one under our seat and the other one in the overhead. This has worked well for me.