For the Bogen 635, you probably don't need a lens longer than 75mm or 3 inches. That would be for 6X6 negatives, the max the enlarger can handle. That will allow you to make 11X14's on the baseboard without reversing the column.
I also use the 75mm for 4X4 negatives
If you are using 126, 35mm, full frame or half-frame, you will probably want a 50mm or 2 inch lens. The 75mm will work, but you may have trouble making enlargements above 8X10 or doing even moderate crops.
If you use the enlarger for subminiature work like Minolta 16, or 110, you may want to look for an even shorter lens.
The shorter lenses will not adequately cover the frame sizes of the larger film sizes, so don't bother trying to use a 50mm lens for 6X6 negatives.
The Bogen has 39mm thread, and the does not have an interchangeable lensboard. That's important if you end up with one of the more antique enlarger lenses with a different mount.
-Paul