It has a diameter of about 12 centimeters. So slightly bigger then a coaster. Since the plaques are cut by hand they are never plane. So I plane them before use with a 1/4 inch end mill.
I do have small machine planer, but these plaques are too small for it.
Iโve seen people make a planer cart for thin wood, just a 3/4" piece of plywood with a waxed top and a cleat to keep it from going through the planer. Of course this is not only thin, but short, so probably not going to work.