It's great to have a nearby rental option. If they have more than one bike/brand in their stable which fits you, try riding each different one -- they do handle differently.
2.54 cm per inch -- so, for rough comparisons, multiply the inches by 10 and divide by 4. I'd pay more attention to the standover height than the frame size, and, if you're buying a used bike, a slightly shorter seattube for the captain is a good thing -- gives you more flexibility for setting the stoker's handlebar height. A shorter seattube for the stoker gives you room for a shock-absorbing seatpost if that seems like a good idea to you.