Somewhere years ago I remember hearing a remark that it takes, and I think I remember this right, 12-15 sq ft of space for every one person you want to fit in a room. This accounts for the person, their seat, table area, and walking room around everything. It all depends on how packed you want them in there and what the building code allows for occupancy. Call you local building inspector and get their take on the space also.
To touch on your questions about how large the room looks on a slow night with 2-3 tables being used and all the rest empty. I guess my observation is still build what your business model warrants and yes you will have nights with no customers and nights with the tables are being turned over 2-4 times each. It all falls down to what your store is doing as far as sales and what you can afford to do. Every business model is 100% unique to its own situation. Hope this helps you out.