In a Charlie Chat, they stated 1.5TB. The limitation used to be 750 because most 750s were a single disk inside the box, whereas early 1TB external disks were a pair of 500s, and the EHD software can't deal with multiple disks inside the same device. Incidentally, this "update" 1.5TB statement was made in a time when 2GB single-disk externals were rare, too. CSRs never know anything meaningful, let alone useful.
1.5TB is also the limit of the TiVo HD, which is based on the same hardware chip that the 612 uses. The TiVo's software can't recognize more than 2TB total (internal + external), but I don't know about Dish's limitation or reason for limiting, if it has one at all. If somebody's tried a 1.5 and failed, I haven't seen it (but I don't read every single post, either). I just like getting my dollars' worth, and 1.5TB is the disk storage sweet spot at the moment.
The info about 722 vs 211 is correct, but the 622, 722, 621, 722k, and [presumably] 922 use the same EHD technology, so if the OP gets a 1TB disk for a 621, it should transfer all of his recordings to a 722k or 922 just fine.