[bbbentley]

RE : Trailer. Build it such that you can pull the trailer all the way through, so you go in one door and out the other. You don’t want to have to back trailers up if you can avoid it. And make that pull through area wide enough that after a long trip or inclimate weather, you can pull the whole trailer and truck inside and shut the door. And the door high enough that a rollback can haul something through the door.
RE: Ceiling. Make the eave higher than 12’. You may want to make a loft and you will be ducking rafters if you only have 12’ and are of average height. A minimum of 14’ to the eave. Keep the building clear span. If you do make a loft and you plan on putting vehicles under the loft floor area, then you will want an engineered clear span floor truss, so no supporting poles are in the way. If you have no plans for a loft, consider having roof trusses made that have the center of the span open. With this approach you would want the doors on each end in the center of the gable. You could build a much shorter side wall and gain several feet of clearance in the center with this type truss.
A building within a building. Large buildings are hard to heat and cool. If the majority of the building will be used to store “stuff”, consider walking off an actual work space that is manageable to heat and cool or like having a smaller building within a building.
Read Garage Journal.com thoroughly and and there are some farm magazines that put out the best workshop ideas in a once a year publication. There are a lot of great idas. One I liked is putting concealed anchors in the floor. You take a small (4x4 or 6x6”) lid off and there is a stout “D” ring or eye bolt. You have to have this really secure to steel before pouring slab. That way if you want to straighter something, like a bumper, you can chain it to the floor and secure it to pry or jack something back to shape. Acts like a frame rack.