The stones that may have been used are still available today. The link below has some info on stone sizes. This list is from records of the quarry where the stones originated from.
http://www.polymetaal.nl/siteUK/shopukwork/enter.html
Lets take the smallest stone on that list, 12X16, this jives with a proof book that I have that has this very stone size written by the printer or prover on some of the proof pages. You could fit two sets of twelve quite nicely on each. twelve cards come to nearly 6X9 in size--the most common size of an inner cigar label. The Wagner proof strip with 5 cards might suggest 10 cards per set and 20 per working stone.
These would then be transferred in multiples to larger stones or metal plates.