Sorry for your loss.
For human urns I've been told to figure 1 cubic inch per pound. For a 200 lb person + or - that would be an inside measurement of 6 x 6 x 6. I've figured that to be a bit less than a gallon size. For Thanatos at 10 lbs that would be less than 3 x 3 x 1.5, maybe a cup, cup and a half. You do beautiful work. I'm sure Thanatos would love being always near you as you play in clay.