Firstly, is there a limit on how big you can go? If not, work out what you want to get in to it, ie, one car or two (if any - some people never garage their cars, regardless!) and what else you will need space for, eg, a workbench, 5 bicycles, a ride-on lawnmower, garden furniture, etc. My advice would be to make it as big as you can possibly get away with (within reason) as it will have much more value and scope for use in the future, and if you find out later that its getting tight for space, it will be very difficult to make it bigger. You will always fill it with stuff you don't want in the house. Also consider planning for a decent loft space for storage. GO LARGE!!