I think the size of the urn varies according to the crematorium, so I would check with the place your relative was cremated for the size of the urn. The volume is not one size fits all, a child or an elderly person with poor bone mass will not fill an urn, whereas a fit healthy individual (until recently) may provide to much bone for the container.

The process unless things have changed is that burnt coffin and other ash drop through leaving the bones, these are powdered and placed in the urn. Quite a neat process leaving the deceased only for the urn. Of course if there is a bit left over this may well join someone who could not quite fill the jar so to speak.

It is a nice gesture to make an urn from the deceased tools and favourite timber, it adds to ritual and leaves a lasting reminder. I completed my fathers monument, something he would have prefered have done himself only Mum would not be party to the process. Sometimes getting things done before the event does not sit well with those closest.

John.