I've made a couple of urns from wood over the years, both turned on a lathe and rectangular wooden boxes.
In reading this thread I've noticed that it wasn't mention on how to size an urn.
You might read on the web that for each pound of weight you should have 1 cubic in volume, sadly this is incorrect. .
After speaking to a funeral director (a couple actually) I learned that it doesn't matter if the person weighs 175lb or 350lb, the cremains are about the same at 200 cubic inches and the standard size container measures 6"W X 4.25" d X 8"H (inside dimensions).
Its easy to layout a rectangular box for the correct size, but throwing or turning an urn on a potters wheel or a wood lathe is not as simple.
The way I check the volume is to measure out 14.5 cups of rice for an adult human, 3 cups of rice for a good size adult dog and less for a cat.
1 cup of rice is 14.43 cubic inches so 14.5 cups = 209 cubic inches. Sorry to be blunt, I feel its better to have a little extra space than to not have enough space.
I'm not experienced enough with clay to give information on how to account for how much clay shrinks during firing but this is indeed something that should be considered.
Hope this helps.