Wow, my words from the deep past. Had to do a bit of googling and playing around with lenses,
Found
this page with some equations and a bit of high school algebra,
With crop sensor, APSc format, cover a 20x24 field of view at:
The 35mm about 3-feet
A 50mm about 4.2-ft
and 100 takes about 8.5-ft
I'd use the longest lens you can given the space and the art size. It's easier to be square to the work when you are back further from it. Also, you need some space to get your lights in without casting shadows from your camera on the art. If doing it occasionally, I'd play with a zoom lens, if you're serious, and doing this often, or need great results for reproduction or enlargements, I'd go with a good prime, likely a macro, for flatter field, and less drop off in the corners,
Let us know how it works out for you