A lot of people say that nobody wishes they had a smaller garage, and I guess that is pretty much true. But I know a guy who has a large home and he wishes it were smaller. He had the house built several years ago when he and his wife were both working and they had their two children living with them. Now he is retired, his wife passed away a couple years ago, and he only has his daughter staying with him. His income is way down since he retired and the property taxes have gone up and they are killing him financially. He certainly wishes he had a smaller home. He could sell his house for a lot of money, but the housing market around here is such that for him to buy another house would cost a small fortune because smaller homes seem to be selling like hotcakes.
Anyway, my recommendation is to build what you need, especially with the cost of building materials being what they are today. If you need 40ft then build it 40ft. If you only need 35ft then build it 35ft. Keep in mind, the bigger it is the more property taxes and the more it will cost to heat and cool the building. I would not count on getting your investment back in the event you sell the property, although it is possible that you will. Also, a lot of people are thinking the housing market is going to collapse in the not too distant future so anything you build now will most likely lose a lot of value in the near future.
I wish you the best of luck with whatever you finally decide to build.