These lovely sugar skulls are just under 3cm, top to bottom, and work in 2 parts.
I suggest that you cut out the shape fist an then press the stamp into the cut out. These stamps were specifically designed to work with polymer clay, however there is no reason that it cannot be used with other types of clay, as long as said material has the same consistency and resistance as polymer clay.

That being said, I still recommend that you use a bit of baby powder (mica powders or corn-starch works too) on the surface of the clay BEFORE attempting to use the stamp. this is a preventative step, to reduce the risk of some of the clay getting stuck in the stamp.
Please be aware that some of the deign elements of the stamps are very fine and can be easily broken if you mishandle or mistreat them (in the case of breakage please contact the shop),If, at any point, clay gets stuck in the stamp, it is best to avoid inserting anything into the stamp itself, but rather soak it in alcohol (70% or higher isopropyl) for a few minutes and then using an old toothbrush to gently remove any clay or debris that might have become stuck in them.

There are a number of different buying options:
You could buy the complete set, which includes 5 different stamps and the cutter, or you could pick your favourite design on its own with a cutter, or, finally, you could just purchase the cutter without any stamps.

If you have any questions, please let me know!