What is the actual foot print for this? I want to have a slab poured and am unsure of the size I need. It looks like the legs extend past the walls

Good question! The assembled size is going to be the 18 ft x 9 ft. Meaning that end to end (including the legs) will be that measured distance. It's always better to triple check though, so I highly recommend contacting the manufacturer Intex to see if they can measure this amazing pool again and make sure it'll fit in your yard and your life! They can be reached via phone at 1-800-234-6839