Might have been discussed before, but is there any length/height/width that gives the best bang for the buck?
In my area, you will have a problem getting roof trusses longer than 44 feet. With 2 feet overhang on each side, a good width as related to your roof line is 40 feet wide..... The reason for this is the length of the truck they are generally transported on.
Most standard plans of a garage are made to make best/efficient use of wood. If you are building it as you go without much of a plan, just go to the box store/lumber yard and look at standard lengths of lumber, and cost out some stuff for yourself. Remember to ask for bulk discounts and they may have certain cuts they are more eager to unload. A framing book can also give you ideas on efficient use of materials.
When selecting wood, consider the insulation you want to put in. 2X6 is far stronger than 2X4, and you can super insulate. Depending on your climate, if you heat full time, it is worth it (and if you are super insulated, you don't have to worry about not heating full time at least to a minimum temperature, as it pays for itself)
My detached garage is 30 by 40 by 13 feet high. 13 feet gives a good height for a lift in the future. It is 2X4 construction, and I heat full time with an oil furnace (about $400 to 600 a year in my cold climate). I wish it was 2X6, but it isn't worth retrofitting now.
However, that being said, I am looking at adding a 30X30 or 30X40 extension onto the back (with a half loft)! LOL You will find that things quickly fill up, especially if you are not well organized, or have other demands on storage (like lawn tractors, and wife stuff, and bikes, and etc etc.)
Ryan