Will I leave behind extra cremains if I am cremated at a young age?

Recently, my teenage brother was diagnosed with rare blood cancer, and his prognosis is not good. He desires cremation, but we are unsure of the appropriate size urn to buy for him after he passes away. I've been looking into urn shapes and sizes, but I'm unsure if a typical urn can adequately contain his remains. Does he need an extra-large urn?