When I built one several years ago, it was suggested that I use 1 ci. for each lb. of body weight. (200 lbs. = 200 ci.)
However, you must also keep in mind the final resting place. We will be place in a columbarium wall at out church so, I was limited to a urn of 6" x 9" x 5" high o. s. dimensions which was less than 270 ci. (when counting wall, bottom and top thickness.)
I've made about 6 of them, 2 of which belong to my wife and me.