I would give us some info on the dimensions of the place you have in mind or have purchased first to see what you have to work with. I think you will get better suggestions that way.
This is the first time I have ever thought about this topic so here is my two cents(Is is 'is my two cents' or 'are my two cents,' because in this situation i believe two cents is actually singular because i am not literally talking about two things?). First I would make it as big as I could. However, I would take into consideration how much stock I wanted to buy because I would want the humidor to be full, not leaving a lot of empty room making it look poorly stocked.
Again, so many things must be considered I could probably discuss this for quite a while. I am curious to see what other more seasoned smokers have to say.