With my Nikor 6x7 enlarger all the way up my 90mm lens prints 16x20" with a 1" border. I need to use my 75mm lens to completely cover 16x20" paper with no border at all. I bought the 75mm lens first for use with 6x6 and then got the 90mm for 6x7 but honestly the 75mm lens really isn't bad in the corners, especially if you go all the way up for no borders on 16x20, then the farthest corners are off the paper anyways. Made me wonder if I'd wasted my money on the 90mm but it is a bit sharper.
It depends on the design of the lens and its coverage. For the average enlarger lens this would be the diagonal of the negative expressed in mm rather than cm. A 60 x 70 mm negative would need a 90 mm lens. Anything shorter and you run the risk of falloff in the corners.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.
Designs basically have a number of lenses. Generally you want a 6 element design if you can find and afford one. There are wide angle lenses which will cover larger paper too. 90mm doesn't have a lot of choice compared to other FLs.
There are some slight differences between enlarger lenses, I have a brand new 50mm Componon-S and also a 50mm Componon from the early 1960s. The old one was cleaned to remove a little haze from almost 50 years of usage in a university darkroom. I did see a tiny difference in resolution. Make a 16x enlargement with each lens, and you can barely see the difference with a 10x loupe when you look at the two prints.
Was it worth buying the new lens. Yes, but not for any difference in image quality. Now I don't have to remove a lens from the auto-focus D659 and put it in the big 4x5 Durst and then fine tune the adjustable lens mount for focus in the other enlarger when I put it back.