So far, nobody has mentioned film format - a 50mm enlarger lens is considered a suitable for enlarging 35mm negatives. Purists have tended to move up to an 80mm in the belief that there would be less light fall off and increased sharpeness. Fred Picker led the charge in this belief - recommending the 80, 100, 135 and 150 lenses for the 35, 2 1/4 sq, 2 1/4 X 3 1/4 (6X7mm) and 4X5 formats, respectively.

While I found Fred to be a wee bit dogmatic in certain areas, I think he was correct here and have used those lens/format match-ups with good success.

If you find, by the way, that you need to increase contrast as you make larger images - you have a fogging problem in your dark room. (either exceeding safelight limits or other non-image light striking your enlarging paper.) Exposure times will increase but contrast will not change - provided you have a non-fogging environment.

For what it is worth.