I think you have a pretty tall stand, so it should be fine. It could be an issue with a smaller stand, cause you would have issue removing the skimmer. I think the extra 4" may help so you don't have issues with overflowing. Especially if you run an ATO. There isn't much volume difference between a 15g sump running level, and the resting level (pump turned off). When setting up an ATO to be flood proof, you are suppose to get your sump running at the proper level, shut the pump off. Once everything stops draining, measure how much room you have left in your sump before it would overflow. That amount is the max capacity of your ATO container. This way, even if it got stuck open and dumped the whole container in at once, then your power went out, it still wouldn't flood. At that foot print (both 15 and 20L), it is approximated 1.25 gallons / inch of level in the tank. So if there is only 2" of space before the sump would overflow, your top off could only be 2.5 gallons. When the extra 4", you sump could hold 7.5 gallons in your ATO.
For future reference just multiply the foot print of the tank in INCHES not feet 24" x 12" = 288, then divide by 231 (cubic inches in a gallon), and it will tell you how many gallons are in 1" of water in your tank/sump. You can also use it to find the total volume. 15 gallon tank is 24" x 12" x 12" = 3456 cu in. Divided by 231 gives you 14.96 gallons. Pretty close!