Enlargers work backwards when compared with cameras (they are like projectors).
The shorter the lens, the higher the magnification.
Generally, you don't use shorter enlarging lenses than the ones that are recommended for your format (50mm for 35mm film, 80mm for 6x4.5 - 6x6 or sometimes 6x7) because the shorter lenses won't cover the larger negatives. By "cover", I mean sharply focus all the detail in the negative right out to the corners. The 50mm lens will sharply render the centre of a 6x6 negative, but the corners won't be sharp, and may not be shown at all on your print.
You choose the enlarging lens based on coverage and you adjust the enlarger to set the magnification.
Hope this helps.
PS there are special purpose exceptions to the above - like wide angle enlarging lenses - but its best to consider the usual choices for now.