Actually, there is no mandatory size for these. There are common sizes, but you can be a bit large if you want. They just should be linen, clean, etc. and a red cross is certainly desirable.
The style of hem can also vary -- unless you're looking to duplicate a given chapel you're accustomed to or something
I haven't managed to acquire this item yet myself. Nor a lavabo dish. The credence table has the other things (cruets, paten).
Some things are harder to get on Amazon.com.