The first thing to decide is what is your principal motivator for building a garage? In most cases, a garage is intended for storing your cars and trucks out of the weather in a protected area. If that is the case with you, then you should first determine what size vehicles you have and what size vehicles you are likely to have in the future. Look at the vehicle size table on the How To
page for some guidance. How many vehicles do you intend on storing in your garage?
Once you have the basics figured out, think about what other things you may want to store in your garage. Do you have power tools for your yard, a lawn mower or snow blower for instance? Do you have bicycles that need a safe place to be stored? Make sure you give yourself enough extra room for storage. You will probably want to have 5 or 6 extra feet of room either in front of, or next to the area where you will be parking your car or truck to accommodate your other items. If you are going to be adding a work bench or gardening bench to your garage be sure to leave enough room for the bench plus enough clearance for you to work in.