I have almost exactly what you are planning on building. Mine is 32x40x11.5 and I love it. However, and this may because its our main garage, but its a little small with 3 vehicles and a 21' boat. I've got room to work but find myself pulling both our cars out to work on things.
You will be able to park (4) longbeds in there pretty easy, but if you decide to start tearing one apart you may find it a bit cramped. I waited too long but tried to find a place to store the boat this winter so I could have more room on the tear down of my truck.
We've talked if we ever move/build we need an attached with atleast a 32x40 but preferably a 60x40.
Congrats btw. Once you get it built you will never know how you survived without one.
Here's a couple of old pics to give you an idea of space and layout.
