The stand I built for my FT-5 R2 is four feet by four feet, because that's the size of half a standard sheet of plywood. It's 25 inches tall, because that is the clearance required to move it, standing on edge, through a doorway in the basement. This allowed me to build the stand in the basement and move it into a downstairs bedroom when complete. It has casters, but mounted on one edge, so that one person can conveniently move it edgewise from one room to another. It also has a power strip with a long cord permanently mounted beneath the lip on one edge, so that I have convenient access to power for use by the printer and other power tools. Such as an electric drill, which I used in the course of making a drybox for filament, and a laptop computer, which drives the printer, and a lamp, which is important for tinkering etc. I also used a hole saw to place a hole in each corner of the stand, and one in the center of the stand, for routing cords. Together with the power strip, this arrangement keeps the cords under the table where they're less likely to be in the way of people and other things.
I would not put casters beneath the stand. There's enough inertia in the FT-5's print head to make the whole thing move, albeit very slightly -- but that would be too much, and likely annoying as well.