I agree with the 60X120, I had one and it was perfect for me.  Looking back, mine was 16' sidewall, and was barely tall enough for my folded 32' field cultivator, but it worked.  I would go 18' tall on another one.

I sold the buildings in 2003, and the new owner needs a bit wider than 60'.  Since I put a 25' door and a 30' door on the east side, and backed my semi into the 25'door, it worked fine for me.  However, the new owner's truck is longer, and he has to unhook his trailer to shed it.  Perhaps a 70' wide might be a better choice.  I liked all the doors on the east side, away from the prevailing winds.

With the 120 foot length, I could drive the semi and the straight truck out of the 25 foot door, and park implements in the space south of the door and more implements in the space north of the door, and drive the trucks back into the alleyway.

I could do the same with the spaces on either side of the 30' door, parking the combine and tractors in the driveway.  You need to plan to park things that can easily be moved in the open areas, and not have to hook on to something to get it out of the way of something else.  By having wheeled mobile implements like truck, tractors, and combines in the middle, it was never a problem to get to any other implement without moving another one.

I had plans to use the space between the two doors as a workshop where tools could be driven in to work on, but never got that far in my plans.