Nice Layout. Yes it looks proportionate if the cars are to scale. A basic car is about 18' long and around 5-1/2' wide. The garage is about double a car length and looks about 40'. I think what throws in all the extra space is going for a 40' wide space instead of only 30'. But the extra space would be nice! You could put a low wall in front of the cars to define the parking area and the work space. The low wall could also have electricity run through it with outlets on both sides. It would allow for 40-50' of additional workbenches, cabinets, tools or what else you could need. Everything doesn't need to be against a wall. But, remember that "Space Creates Vacuum" - Create that much open space and it will be filled with stuff to trip over. You will always fine enough stuff to fill the space you build. Just ask anyone on here.
For a couple other suggestions... Consider swinging the bathroom and compressor room doors outward. Since they will be closed most of the time, it really doesn't matter. It also gives more room for storage that will be put in their eventually. Also, if something falls over in the room, it won't block the door from being opened.
The amount of space between the cars is nice, but doesn't give much circulation space on the ends to access the workbench or shelving. Right now between your 12' doors, you have 5 pieces of wall at about 2.4' long. If teh doors were pushed together, that wall spacing could cut down to 16" and you could have 2' more space on each end to work. Or consider going with only 10' doors instead of the 12'.
And I agree with the others, Floor Drains and Beer Fridge!
Good Luck with your project.