The tip is checking the body weights in pound (lb.) which will roundly equal to same value of the volumes in cubic inch of cremains and if this value multiply by 16, it will converse to the volumes in milliliter (ml.), then check these volumes in cubic inch or in ml. on each product capacity. This same way applies to check for pets also.     

* The important measurement

1 pound (lb.) body weight ~ 1 cubic inch ash 
(1 pound of body weight will yield roundly 1 cubic inch of ash)
1 pound (lb.) = 0.4545 kg.
1 cubic inch = 16 milliliters (ml.)

This chart is an idea to select the correct urn size for whom with the volumes on each category.

The adult urns for average weight should hold 200-220 cubic inches (3,200-3,520 ml.) and the extra-large adult urn for whom over average weight should hold 220-250 cubic inches. (3,520-4,000 ml.)

The companion urns for two love person combination should hold over 400 cubic inches. (over 6,400 ml.)

The child & infant urns for under 13 years old should hold range about 30-200 cubic inches (480-3,200 ml.) from their weights.

The keepsake urns or miniature urns are designed to hold a partial amount of ashes with lower than 30 cubic inches. (lower than 480 ml.)