Any tips building/designing a 28x28 Board and Batten garage Gambrel Style roof?

Hi, I am getting ready to build a 28 x 28 Garage, it'll have board and Batten siding a Gambrel Style tin roof, I am going to have a 6 inch concrete floor with two foot high knee walls, the garage will have 10 Ft ceilings on the first floor and 8 ft on second floor.

I plan for now to have either one slider or two sliding barn type doors in the front and a regular door on the side toward the front of the building.

I plan to tyvec the outside of the building and then put up the board and batten, trusses will prob be 2ft oc.

The garage will be build into a bank a bit and that is the reason for the knee walls. There not be a beam in the center so the front will be one big opening, any idea how big I should make that?

I plan to have a Hay door up top with the over hang and pulley system like the old barns.

thanks