How do I figure out what size urn I'll need for my father's cremains?

My father just passed away recently, and I was put in charge of finding the perfect urn to honor him. Dad was always a bit different and didn't want to be buried. He often joked about us forgetting about him since we all moved out of the house, and I remember he explicitly wanted to be cremated. It’s a bit of a challenge to pick the right one for the big old man. I don’t know how to calculate the right size of the urn I need. Can anyone help, please?