Hey mate, i have one of each in front of me. Also if you can find a manual, it's on page en-76. Looks like the same for rb20-25.
Measured with a tape measure (cause thats all ive got)
Smaller one is 5mm thick, 11mm inside hole and 22mm outside diameter.
Larger one is 5mm thick, 11mm inside hole and 24mm outside diameter.
Hope that helps