I've got the standard size
like this
However, now Harvey is a fully grown female hamster weighing in at just over 200g, her bottom tends to stick over the edge so it's not as successful as it once was! This is possibly why she's taken to using the 500g Flora tub that was her sand bath.
I've just bought a Pyrex oval pie dish from Sainbury's which is 21cm x 13cm, it was reduced to about £2.50 and I'm planning to use it as Harvey's new sand bath but that would be a decent potty size too.
Edited to add, that I started with one of the enclosed pottys with the plastic base and the clear/sparkley plastic lids. Total waste of time, Harvey was getting stuck in that when she was only about 3-4 months old.