M21 headlight question

[Steve 1946]

Hi all
I'm new to the scene as this is my first project and I'm asking for help about the minefield of headlamps.
My M21 1946 model has an 8" headlamp but I've been told it should have a 6" headlamp which I have looked into and read numerous of pages.
I have spoke to bsa motorcycle supplies today and they have said both have been used. So any help would be appreciated. Thanks