I don't own a shop, but I've worked at one, and been behind-the-scenes in others. Multiple floors is a bad idea, but extra space is absolutely necessary if you intend to have a healthy repair business. Also, even if you think you have plenty of space, you NEED a policy for the timely pick-up of repairs. Otherwise, some broke and/or inconsiderate ppl will leave their junker there FOREVER. The guy who owned the shop I used to work at would call when the repair was done. There was a 7 day grace period, then the storage fee of $5/day began. After 1 month, the bike was up for sale and proceeds would go to paying off the repair. We had every customer who left their bike there sign off on this on drop-off; prior to that, some jokers left their bikes in the crowded little shop for over a year.
But, yeah, it sounds like you'll need space, but more of it for the repair shop than for retail merchandising. If you don't have competition in your town/area, prime real estate won't be necessary. If there is competition, remember: location, location, location.