From what I understand, the Crotalus Enyo (Baja rattlesnakes) stay relatively small (3 feet-ish), but they are pretty hot, a lot more so than a copperhead, so you should consider that as well.
They're kind of difficult to come by too, unless you really do some homework.
The Sistrurus family are great. Dusky pygmy's and massassaguas stay nice and small, but wouldnt really be considered a "True" rattler. These have been Level 2 for me after starting off with a copperhead.
To start off, I still recommend a copperhead, because they are accessable, and.....well, how do I say this correctly......a bite from a copperhead is a "low threat to human life" if you catch my drift. Im not saying it's a joke to get bit, because it's not, but it most likely will not cause major limb loss or death.
I started off with a pretty laid back copperhead, and it's done me REAL well as an introduction to hots. I've moved on to pygmy's now and dont imagine I'll ever move beyond them. Im tickled to death, pardon the pun, with the Sistrurus and plan to just expand my collection of those and some nice Laticinctus (Broadband Copperheads).