Make them at least a foot wider (six inches each side) than the largest pot you have. Also make the table at belt height, at least, not any lower. Also dig at least 24 inches down to plant the post. Anything shallower and you could have smashed trees one after noon as the post lists and is overtaken by top heaviness.
You can also build in a turntable using lazy susan hardware you can buy at Home Depot. Turntables make turning larger trees a lot easier and safer. Spinning a large tree for even sun exposure can torque posts severely and break them -- I have larger trees, so I've learned all this the hard way.