Cartridge CASE length is 1.98 inches
Loaded cartridge OVERALL length is exactly 3 inches, according to TEXT BOOK OF SMALL ARMS - 1909.
This should get you going. If not, PM me or post something here.
You might mention to your friend that he would likely be better off to use SR-4759 for a Black substitute. I have tried IMR-4198 in my .50-calibre Remington and, even with a wad, there is just no way that it burns anything at ALL predictably. Generally, there is half-burnt powder from one end of the barrel to the other and a squib shot with the bullet wobbling slowly downrange at a little better than 14 feet-per-second. The powder is just too slow for that rifle.
I have a 1961 Lyman book here and, back then, they still were loading for a lot of the old black-powder rounds. The preferred powder always seemed to have been the SR-4759. This is a SPORTING RIFLE powder and was designed originally as a replacement for Black and it is related very closely to the old-time Bulk powders. As it was made for many years, you could almost use it bulk-for-bulk, but I believe that it has been 'remodelled' recently as to grain size, so now you must go by WEIGHT of the charge. To use the SR-4759 as a substitute for Black, you use THIRTY-EIGHT PERCENT of your Black-powder charge BY WEIGHT. Generally, this will give you the same velocity and the same pressure as if you were shooting Black, but without the half-acres of smoke every shot and all that danged fouling to dig out of the bore. It burns very clean at this charge level. It is safe in the old .43 Mauser and was specifically recommended for the Mauser cartridge, so should be just what the doctor ordered for most of the old ones.
Still, I don't think I would run it in a Snider.... a lot of them had soft brown-iron barrels and I really don't trust anything except Black in them, and at fairly low charges, just for good measure. Kynich used to load the Martini-Henry with smokeless, but even they didn't load Snider ammo with it. They likely knew what they were doing; they made the stuff for over a century.
So, have fun!