Go bigger than you think you need. I have a commercial building with a big overhead door. Recently, there was a hail threat, and I decided to put our Bambi inside. Visually, I thought it would be no problem. Actually, it was tight, width and height. Fortunately, the Bambi is not a wide body, or it would not fit. Plan for the future - you don't know what size your next A/S will be, but it will probably have a rooftop unit. You never know....
It's funny, the Bambi looks small compared to most other units, but inside a building it is HUGE.