My building is 52' x 24' x 16'. My door is 20' x 14'. I wish I had your width as mine is tight to maneuver around when my slides are open.
If I were you, I would go at least 52' deep. If you don't, you will have difficulty keeping your truck hooked up to either your boat or your 5er. My 5er is 32' long, my ski boat is 25' long (with trailer tongue & outdrive, and I have a long bed crew. I could not keep my truck hooked up to either my boat or 5er if I went with 50'.
2012 RAM 3500 Laramie Longhorn DRW CC 4x4 Max Tow, Cummins HO, 60 gallon RDS aux fuel tank, Reese 18k Elite hitch
2003 Dodge Ram 3500 QC SB 4x4 Cummins HO NV5600 with Smarty JR, Jacobs EB (sold)
2002 Gulf Stream Sea Hawk 29FRB with Honda EV6010