If you've got access to a copy of the Standard Catalog of World Coins, it's got a handy "Chart of Coin Sizes by Millimeter" with circles ranging all the way from 10 mm up to 79 mm, along with a ruled scale of 24 cm.
Me? I'm lazy and just slap 'em alongside a ruler to check the diameter.
Don't see why not, provided the pic or printout is indeed actual size.
By the way, I forgot to mention that you can pick up plastic vernier calipers for around $2-3. They're intended for student rather than professional use, but will still get the job done.