In the regular stam there is only the larger size and the baby, but in the first season there was a really small version as well. And there are other slightly different variations that still look almost the same - such as the Mina stam which is the same height and width as the regular stam, but flat rather than squishy (in the same way as the baby stam is) and with just a chain, no handles. And I think there was also another size similar to the baby stam a couple of seasons ago, but essentially the same.
I think now they are mainly just released in different versions of the regular size (regular quilted, studded, different chains, etc.).