Small case alternators are found on most 2, 3 and 4 cylinder diesels and are found on the majority of recreational sailboats up to 50 feet long.  Large case alternators are found on larger diesels of 6 cylinders or more, usually on power boats.  Large case alternators tend to be the size of a gallon paint can, small case alternators are the size of a smaller paint can

Here are the four different mounting types commonly  found on marine diesels, two variations of the single foot mount and two variations of the dual foot mount.

The single foot mount can have a foot that is either 1" or 2" wide.  The alternator in the picture above has a detachable rear part of the foot so it can be used in either application

The dual foot mount with 3.15" spacing between the feet is most commonly found on Yanmar engines and so it often gets called the "Yanmar Mount"

There is also a mount where the feet are 4" apart and that is called the J-180 mount

Pulleys for V belts come in  Single V and Dual V configurations.  Serpentine style pulleys come in 6 groove, 8 groove, and 10 groove configurations.