I used to own a standard DP TIE helmet. I believe it was undersized when compared with a "movie sized" helmet. But I've never seen a "movie sized" one in person.
The lenses were sort of hard to see through, they distorted your vision to a degree.
There was no padding or anything inside. It was just a hollow shell. So you'd have to devise some way of making it sit on your head in the right position.