Wide enough to put two cars in it?
Ideally you want at at least 11 feet per car. Using a eight foot wide door for each. Not aware of any special requirement however. So I would not make it any smaller then 22 foot wide. Ideally 24 x 24 foot garage size overall would be better.
Remember its not really that much more to add one or two feet when your building, but not having enough room will cost you even more.
Look at friends garages. What is in them. Most likely no car but a bunch of other stuff making its way to the garbage or being stored during the colder winter months. Take this in to account on how big you want to make it.
Woodbridge has some great tips there. In my neighborhood only about 1/3 of the homes have garages...Yesterday I was dropping off my girls at the ex's...and in that neighborhood all of the Homes have garages as part of the overall home design, in better than 50% of them all of the car sit in the driveway because the garages are so full of other stuff...This was one of the first times I had dropped off the kids on a Monday night...that neighborhood has Tuesday AM trash pickup so as I drove through many had there garages open to move the trash to the curb...just another example of people having way too much stuff...
In an ideal world a 24 x 24 sounds perfect...any "increment" of the 4 x 8 plywood sheet size makes for an economical size.