If I’m buying or making my own urn, how do I know how big it should be? If I’m buying or making my own urn, how do I know how big it should be? September 1, 2015 Erika Peterson The general guidelines are one cubic inch per one pound of healthy weight. Newer I’m going to be extremely emotional the day I take my pet in to be euthanized. Can I make arrangements in advance? Older Do I need to buy an urn from you?