How do I figure out which size fork mount I need for my bike?

I own a REI DRT 1.0 MTB. I cannot figure out what size the fork mount is. I am wanting to mount the bike to a fork mount on my roof rack, but don't know which size to go with.

Would any fork mount work?

Alternatively, is there something I can measure on my own to figure this out? New to the MTB world and just trying to figure this out.