I've been drawing homes for a living in Washington state for 32 years.
If given a decent lot size and no real input from a client I'll default to a 24'x24' attached garage in most instances for a 2-car. If there are more limitations I'll make the garage as large as possible, i.e. - 24'x22', 21'x24'. If a client tells me they have a quad-cab pickup I'll try to make the minimum 24'x26'.
Whenever possible I go wider, deeper, or add an area to form an 'L' shape garage so that there is room for bikes, freezer, garbage cans, storage, workbench, etc. A third car bay can be offset sometimes to help add extra space. The plans and 3D models we show clients have recognizable sedans and SUVs shown in them so clients can get a sense of size. In almost every case the client has appreciated the extra room in the garage (since they did not consider the importance initially) or asked for even more room be added.
Tight lots, remodels, single-car families can vary. A tight lot 2-car could be 19' wide, but we all know that it would end up as a single car + storage, or even all storage. A single car garage ideally would be at least 13'x24' but could go down to 11'x20' if the client has a small car. I recently did a 'garage' addition that was 12'x18'. Not that usable as a garage, but the client wanted to use it as a potting shed she could back her car up to and unload soil, plants, etc.