MY Two cents ... I have a 6' x10' layout on casters.
The layout has a 2x4 frame,4 3' 2x4 legs, 2ea 2x4 45 deg angel braces on leg that connect about 1/3 down the length of the leg. The casters are 2" hard rubber, swivel type and are screwed into the bottom of the 2x4 leg with 2 or 2 1/2 long screws and pre drill the holes. I used drywall screws and they have worked for 15 yrs or so however they are not meant to used for this application..try to get 3/16 diameter screws better suited for yours...... My floor is basement smooth concrete with 1/4 " jute backed indoor out door carpet so the wheels do not sink in and it works fine. like the guy above said bigger is better, how ever my 4 hole caster plates are the almost the same size as the bottom fo the 2x4.
You may want to add 2 more legs in the middle with 3 45 deg angle braces because of the length.
A lot of good advise in the responses you should be in good shape, It is useful feature to be able to move the layout, I am glade I did it.
Good Luck,