1) Greater than 50GB, and smaller than 2TB.
2) Your 622 will reformat your disk as some number of 500GB partitions in ext3 format. I suppose, after this happens, you could potentially use Linux (or any O/S that understands the ext3 file system) to store additional stuff on the disk. I haven't done this and I'm unsure if the Dish receiver would find and delete it. But I think that would likely work.
3) There is no longer any activation fee for the EHD.
4) I prefer WD green drives because they're fast enough, use less power, and aggressively go into hibernate mode and spin themselves down, thereby not wearing themselves out. They also come right back out of that mode if the Dish receiver needs to access them again.