I think you will find it has a similar root structure to the branch structure. No major 'tap root' just many large roots that spread out. I think it will be a grinder job.

Hello Barbara, Seaburngirl is right the root structure would have been the circumference of the branch spread. I know your husband likes a challenge, the wider the hole is away from the trunk will give him a better chance of getting most of the roots out, any left in should be treated with a good fungicide, or you could burn the trunk and roots in the hole, afterwards you will need good topsoil to make good the growing area on top.
Or you could always get your neighbours to help. Lol.!!!

I can't see that happening, Docbob! Thanks for the advice - it looks like lots more hard work...the roots we can see look enormous!

Yes, I agree - that's exactly what we do when we have to remove anything. We spent two whole days cutting off all the branches, then OH used his chainsaw and cut the stump back to about 4' tall.