I agree with the poster above - you'd do better taking a friend with you who knows bikes and buying a quality used one from Craigslist. The size depends on your size.

If you have a Cannondale touring bike, why not use that? If you;re 6'2, I'd GUESS you'd want a diamond frame of about 60cm square on a conventional diamond frame, but I don't know how that equates to other frame types.

Any bike can get stolen, it's more about opportunity than quality. Most bike thieves don't know the difference anyway. I'd use a quality ubolt and cables for the wheels...google locking on youtube.

BTW - I worked at a Wendys when I was a teen and they DID let me store the bike in the back storage area. It can't hurt to ask. Of course then you have to worry about other employees taking it, or leaving the back door open.