You can get 512 for under $100 now and go all the way up to 2TB if you want.I think 512 was probably the biggest I saw back when I was looking, but most were smaller than that. It's been a year or more though since I was looking. You lost me at NAS and RAID. I've been a mainframe programmer, but I'm just barely PC-literate. I remember thinking that 400 Mb files were HUGE!
Redundant Array of Inexpensive Disks. And if you want the redundancy part you do want disks of the same size at least.RAID is Inexpensive Disks or Independent Disks. There is no requirement for them to be identical.
NAS is network attached storage. You usually plug a few spinning disks into a device and connect it to your network. Computers access the files remotely. When using multiple disks and the redundant part of RAID if one fails you simply replace it and move on without loosing data. There are of course catastrophic cases but they are low in probability.