It is a lot less expensive to build bigger the first time than it is to have to renovate.
I would recommend a 14' door. Should be able to get any future purchase in it.
If you can make it long enough to keep truck/5er attached, there are advantages to that.
My original building was 24' x 52' with 16' sidewalls. I then enclosed one side and added about 12' x 30'. I wish I would have made it wider still, as it is nice to be able to have the slides out when it is in the garage, and still be able to move around inside the garage easily.
I have power outlets every 8'. And (I think) 16 LED lights in the main garage.
Sky lights and lots of windows for natural light.
Insulated so it can be heated in the winter.
The area where the utility trailer is where I did the addition.
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