I have a 6×12 now and it works perfect for 1 kart. If you want to haul more you’ll need a stacker stand or stand them up. Mine is a v-nose with a workbench up front and a bunch of Pit Pal products. Whatever size you buy, you’ll fill it up and want more space. If I get another trailer it’ll be an all-aluminum 7×16 set up like this:
Get everything you want installed from the start. Wire it for lights/outlets/shore power hookup and get a quiet portable Honda/Yamaha/Briggs generator. Finished interior and rubber coin floor is nice but isn’t essential. An extended tongue is great for maneuverability in tight paddocks. Skip the awning on the side and just go with EZ-Ups. If you don’t already have a compressor and air tank go with a nitrogen tank setup. Put E-track down the sides and recessed d-rings in the floor.