Its a size Medium Wide. I have cleaned it up the best I could.
Have a look at the pictures, what you see is what you get. The item "MBU-20A/P Oxygen Mask for an HGU Pilot Flight Helmet Dated 08" is in sale since Saturday, June 2, 2018. This item is in the category "Collectibles\Militaria\Current Militaria (2001-Now)\Original Items\Hats & Helmets".
The seller is "csardude" and is located in Victor, Idaho. This item can be shipped worldwide.