I built a 30x60 with doors on the gable ends only. Yes plural, I made it a drive through barn. With 60 feet of length I could hitch my full size crew cab truck up to a 30 foot long trailer and still easily fit within the 60 foot length. Don't need any more length but 30 feet is as skinny as I'd go.
Lumber is available in all different lengths now and any cutoffs are relatively cheap. Don't worry about hitting an increment of 4, 8, or 12. It just doesn't matter.
Most important is having doors no skinnier than 12 feet. With a 30 foot gable end you can get two 12 foot doors in plus have a couple feet on each side of each door to make it look good.
Absolutely do not go skinnier than 30 for the shop with gable end doors.