How big of a spiral ham would you get for 20 people? I am thinking of using my 18 quart roaster oven to cook it in. Have you ever done it. The "little" instruction manual that came with the oven isn't worth the paper it is written on. I have been looking for a site to get recipes for the large 18 quart oven size and have not had any luck.
I think you would need a whole ham for that number--I'd imagine it might come in at 12#.
The roaster oven should be fine to "cook" it in. They are already cooked so basically just need to be warmed through and (if it is the supermarket variety (not Honey Baked or a "name") to melt the glaze.
If I recall the directions correctly they usually say to mix the glaze and put it on. Wrap the ham well in foil and heat for 2 hours at 300*. Follow the directions on the ham, not the roaster. The roaster will act like an oven just fine, I think.
I'm not sure what the occasion was last year - but we got the store brand sprial ham - found a recipe for the glaze and we used a propane torch to do the crackle glaze after the ham had been heated through. It was so good that there was very little leftover! It was fun too!