My standard tile sizes are currently
10.7cm (4.25 inches) square and approx 5mm deep.
15.2cm (6 inches) square and approx 6mm deep.
However I can print a much wider range of sizes provided I can get hold of the blanks required.
Some other possibilities:
20cm x 20cm
10cm x 15cm
NB If you've bought sample tiles before putting in a bigger order, please let me know! At times the depth of the blanks varies - which means you don't want to have a mixture of different "sources" of tiles. If I know the exact size of your samples I can make sure the rest of the set is made to match.