lots of good ideas here, if you have alailable funds, but remember, you can always add to your project latter on some items. The size and height is most important now. Only you can make that decision. I would like to add a couple more ideas to the pot. Underground power to the garage, you never know what you might drag home. Get a electrician involved, you may want a separate service from the house. As stated by someone else, you will want a floor covering before moving in and I would recommend an epoxy finish, as in Backally post #45. At lease 12' high bay doors. Attic or above storage to give you more floor space. Again, it's up to you if you want to add stairs or a pull down ladder, which can be installed latter.
