I built a 6x12x3 high from an old shed I picked up for nothing. Had 5 tilt up doors and a basic pedestrian door. I resheeted to look new.
I rejigged the frame to fit a 5m roller door, bumped up the overall height by 200mm to fit over my block wall. I cut into my block a few meters and blocked it up as retaining.
As for bench space, I built a bench that spanned the 6m depth of the shed. Hardly notice it's there if you want to pull a heap of cars in.
If you have a choice, go at least 7m deep. The extra meter makes a big difference moving around when it's ful
I'm also a big fan of skylights. One panel per 3m is great