This may help you some. It shows pictures of various pulley's used on different models, gives pulley diameter, and groove width on them, but not the total length. I guess you could figure that one out. And, of course, the smaller the pulley to the transmission sheave, the slower the ground speed. If I remember right, the manual says not to run the cutter bar mower at over 3/4 throttle. The smaller engine pulleys were 1-1/2", and the 5hp models like my 917.575120, aka Big 5 pulleys were 2-1/2' & 2-5/8".
We have a pretty decent DB page on Facebook, if you happen to dabble on there. Quite a few knowledgeable guys on there. One of them put this page up a couple weeks ago, so I saved the link to my favorites, for easy access.
There also used to be a site up someone posted on the Yahoo DB group. It had every tractor manual, and implement manual on there. Not sure what happened to the fellow that had it on a separate site, but it was taken down about a month ago. One or two of the guys on the FB DB page have it on CD, and are looking to put it up somewhere, so it will be accessible again.
Hopefully this site will help you out. There is also some other useful info there. Home