Hi, and welcome! We don't have the FB, but our 2008 23' Airstream also has the shower located in a back corner (you can go to the airstream.com website and view the floorplans).
Your question is a bit tricky because, as I am sure you noticed, Airstreams are not square. The highest point in the shower stall is where you enter, and that is 6 feet. The ceiling tapers out and down from there as it follows the roofline.
Because the shower head is removable and hand-held, and because there is a seat to sit on, the amount of space in there seems perfectly adequate to us. I am 6'1" tall and weigh 180.
We thought we might not use the shower much, but quickly changed our minds with experience. Hope this helps........