Ok, I am getting ready to make (or at least thinking about it) a cake of a church building. It has the main building part, the steeple, and covered walkway with arches around three sides. Originally I was thinking 16x12 (three tiers stacked) for the main part of it, and doing the steeple and walkway and maybe the roof in gum paste.
So my problem is I think it will be too big. I need it to feed 150-200. Is there a good way to determine scale? Is gum paste the going to work for this? --I've never used gum paste before. How should I do the roof?

Or if anyone else has a great idea for a sanctuary payoff party cake. . . I'm open to ideas.

I like your plan so far. Will it work? I've never done anything like it but I think it would
As for being too big....you say:"16x12 (three tiers stacked)"... I think you mean 3 *layers* for a heigth of 6"x12"x16". This will not be too much at all - in fact maybe a bit less than you want.
If you put one of those layers on a cake board the same size as the cake it can be served in 2x2x2 blocks (4 while the rest of the cake would be served in 1x2x4 pieces (106). This is the same amount of cake - just shaped differently - a total of 154 servings give or take a few.