I ran a 10'x5 with 2' sides for half of my first season. I built an "upper deck" to carry a push mower and 36" walkbehind and had a 52" walkbehind on trailer deck. Sounds bad but with the racks on, it almost looked like a factory job...haha
I had a custom 18'x7' built with 2' sides and a 5' side gate. Still have it along with a 7x14 and a box truck and a 7x12 dump. Its almost like all of a sudden you wake up one morning, have all this stuff, and are thankful your head is attached b/c otherwise you would probably forget it while you run out the door to get everyone and everything ready for the day!
18'x7' with 2' sides is my strong recommendation. Good luck