lifted from the net...
Enlarging lenses, like regular camera lenses, come in different focal lengths. You want to select a lens that will work well with the negative format you are using. If you select a lens that is too short, the projected image will be very wide and you'll have the enlarger head down low to make small-magnification prints, and the bellows may not collapse far enough to be able to focus the image. If the lens is too long, the enlarger head will have to be pushed high to make large magnification prints, and the bellows may not extend far enough to focus the image. Also, a given lens can only cover a certain size negative. You can put a long lens on a small negative, but a short lens may not cover a large negative.