I would suggest always double the size you think it should be. (2-car, make it a 4-car - 3-car make it a 6-car) with enough room for parking the cars next to each other with the doors fully open and still being able to walk between them etc. Plan room for storage and extra storage. Plan out the work areas where everything will have a work zone and will have easy access (if you plan on doing any major projects in the future that is)

And plan it out so you can have the finished toys near the lounge area for viewing, but out of harms reach. Plan the flow for how you imagine all the various uses. Like a drive through rear door area with covered storage in the back for the snow rigs or other toys etc.

Personally, something I would do is have 2-3 work zones. Build/tear down/body area with provisions for a lift (Work and storage potential) A clean zone for detailed work, engine building etc) and another, enclosed area for body and painting.
Then an over all common area for entertaining with a built in grill (if you will have a gas heater, why not plumb for the grill) and yes, a fully stocked bar area... bathroom with shower all the usual amenities...

Some time when you come down, I'll have to bring you to Griot's Garage: http://www.griotsgarage.com/ Richard Griot himself has a dream Garage, but the company he and Mark Greene built is even more impressive. Every item they sell is top of the line and or part of the product line they developed and produce themselves. But they have the most innovative things to help deck out your Garage once you get to that point! From flooring to wall storage to tools...

Can't wait to see what you come up with!

B!