Here is a quick and helpful video about cremation urn sizes
Most people need a standard adult urn which generally are sized to hold the ashes of a person who weighed 200 lbs. or less prior to cremation. Cremation urns do come in extra large, urns for two, medium/small (a portion of ashes), and keepsake sizes.
A good starting point is to find out what your loved one weighed prior to cremation. Each pound of weight will equal one cubic inch of ashes after cremation occurs. Thus, if the person weighed 150 pounds prior to his/her death, then you will need an urn that has the capacity to hold 150 cubic inches or more of ashes. The majority of our urns hold 200-220 cubic inches of ashes.
Keepsake urns, or mini urns, are popular to hold a small amount of ashes in remembrance. Keepsake urns and medium urns also are used when family members want to divide ashes among several people.