I’m waiting on this info before I order one.
I want to use either 40mm or 42mm external rims and I think it’s pushing it 😁
It depends on your personal clearance tolerance

, but I think those 40s is a bit much.
For what I was able to measure thanks to frame drawings that was posted a few pages back. I measured frame width 46mm so with 38mm tyre there should be clearance of 4mm, you can calculate what that clearance would be with 40s+ tyres.
There is a loot more clearance in forks so maybe go for wider tyres and rims for front and a bit narrower in back.
Disclaimer

I can not guarantee my measurements are absolutely correct, but if that drawing is right I have good experiences with this kind of measurements.