With a diffuser type dichroic head you can "often" use a slightly wider lens than normal, assuming that it is designed for that purpose of course and that you avoid being unlucky with the diffuser design (there must be some models where there is noticeable fall-off). The infamous Focomat V35 is fitted with a 40mm lens and works admirably, he wrote, smugly.

There are other limitations though. You have to be able to focus the lens for all the printing sizes you may want to do, else coverage won't matter at all. Also, the horizontal distance between the vertical axis of the lens and the enlarger-column must be enough for you to fit a bigger easel in there, at the increased size you want to print - if there isn't room on the baseboard you may (depending on focussing again) be able to turn the column round and use the floor instead, giving possible alignment issues and also happily removing the need for having a wider lens . . .