Two things. if you need to know the size of a nut and can get an adjustable wrench on it you can measure the gap in the wrench's jaws to tell what size socket you need.

Second, I never use an impact to loosen a strut nut, no need. Just loosen the nut first thing before you jack the vehicle up. The weight of the vehicle will hold the shaft in place. Loosen only, don't remove it. yeah an impact makes it easy but if you don't have one it's not a show stopper.