This is a WW-I German helmet which appears to be in relatively good condition for its age. It has light rust on the inside, mostly around the metal portion of the liner. I could not find any markings in respect to the size of the helmet stamped in the rim.
The picture with the measurements may give you an indication of the size of the helmet. As you can see from the pictures, the original liner is present but in very poor condition. The item "Original WW 1 or WW 2 German Helmet" is in sale since Saturday, March 24, 2018. This item is in the category "Collectibles\Militaria\WW I (1914-18)\Original Period Items\Germany\Hats & Helmets". The seller is "johredde_28" and is located in Tucson, Arizona.This item can be shipped to United States.