My garage is 2 cars wide and 2 deep, though not as wide as yours will be.
I'd suggest:
A side door so you don't have to open the front one when it is cold outside to get in the garage.
Put up a curtain between the front and back sections, so you can limit both heat loss and dust when working. Especially if your going to have some machinery like lathes and mills.
compressor ideally in a sound proofed enclosure, and with fixed plumbing and a water drain in the line.
extra lighting over any fixed bench areas.
allow for some racking for items you need often, and a mezzanine for the bulky stuff you only need once in a while (and fill it up before the wife does !!
)
A parts washer.
A decent sized trash can.
As well as the bolts sunk in the floor suggested above, some ground anchors so you can chain items to the floor if required.
A very good alarm system, ideally with a camera link so you can see what goes on from indoors (a friend has his camera linked to his video recorder, so he records any activity when they go out).
Not only a toilet but a sink and a fridge, so you can either make tea/coffee, or keep the beers cool
I use an infra-red heater (heats you, not the air). Works great, and is cheap to run. Floor heating is better, but costs more to run.
And finally, a whipping boy to do all the work while you drink beer and watch them work
Hope that helps