Depends, most from 35mm scanners will be around 40mm.
That would be perfect focal length. Hmm, where can I get one ..
I use an APS-C (Fujifilm XT-2) + an enlarging lens on a Durst slide copier to digitize 1/2 frame 35mm up to 6x6 negatives (6x6 is pushing it a bit). I mainly digitize 35mm and some 6x6. 1/2 frame 35mm I used a 50mm enlarging lens (had a roll or two worth). For full frame 35mm and 6x6 I use an 80mm enlarging lens. Took some adapters, but I got it to work pretty well. I also use a negative holder, a Pixl-Latr, and some enlarger negtaive holders. The Pixl-Latr, though far from perfect, works best overall.
It was not only few days a go I got the idea from here, probably from you. If it was you or someone else: thank you, this was so eye opening!
My bellows+slide copier is this one:
It is a bit of a squeeze to get 35mm frame to fit with that setup + 50mm lens on APS-C sensor. The first bellows between camera and lens are just a tiny bit too thick. I could remove + modify the setup but I plan to use to to make positives. I think I paid 100 euros for the setup. It is so good, everything is built with accuracy and it is heavy + stiff.
Full frame body would be cool and help fitting everything with that setup. Maybe some used full frame Canon DSLR, I need to check prices.
Still pretty hyped about this. I have never got so good digitized material from film.