I’ve been wondering this, also for this I want to know the smallest in Crotalus. Not Sistrurus, if I were counting Sistrurus it is the Pygmy Rattlesnake. I’m guessing it is most likely the Banded Rock Rattlesnake (Crotalus Lepidus Klauberi).
No reason in wanting to know, just feel like knowing.
Also this a screenshot of the one I think it is.(photo is not mine) Which is the Banded Rock Rattlesnake
The runner-ups are either the ridge-nosed rattlesnake (Crotalus willardi), which has a maximum length of 600mm, which is only a centimeter smaller than the klauberi, or the twin spotted rattlesnake (Crotalus pricei). The pricei can technically grow a few inches longer than the other two, but is usually only a little longer than a foot. It also looks just like a nightsnake, so despite how rare they are, if I ever get bitten by a rattler, it’ll be this one because I picked it up thinking it’s a colubrid.
However, our winner is…
The Mexican small-headed rattlesnake (Crotalus intermedius). Maximum length of 570 mm in the wild, only a tiny bit smaller than the willardi.
Look at this little goober. Potentially lethal, nonetheless. Someday, when I start up my venomous collection, this one will have a place. It’s quite rare, and lives a continent away from me. Normally that’d be enough to dissuade me from something venomous, but CroFab should work, so the only thing killing me would be the hospital bill 
Fact checking myself here. I missed Crotalus transversus, with a maximum length of 465mm. That’s significantly smaller than any of the other species, so I’m going to need to hand it to them. Not quite as pretty, but a snake that small’s always going to be adorable. One of these days I’m going to die petting things which don’t want to be pet 
It’s part of the same complex as C. intermedius, along with a half dozen other species. They’re all grayish-brown, they all have small rattles and heads, and they’re all from the mountains of central Mexico. I wouldn’t be surprised if any of the other species are even smaller, but some of them have only been recorded a few times, so I can’t find anything which gives any exact sizing data on them. As such, transversus wins the gold. They probably can grow slightly larger as only about 20 specimens have ever been collected, but probably not by much.
I think that snake is even smaller than the Pygmy Rattlesnake
Yeah. The pygmy reaches something like 28" at full size, almost 10 inches longer than the C. transverus.
