FWIW - I have two Netgear ReadyNAS units at home .... one a two bay, the other a four bay. I have had good luck with them .... easy to use and configure ... and these have been going strong for, what, 4 years or so now (knock on wood
) Technically speaking, the two bay is configured as a RAID 1, the four bay is configured as a RAID 5. There are reasons for doing one or the other, which is a MUCH longer discussion - I would STRONGLY recommend you back away for just a little and get on the web and read about RAID, and what are the benefits and disadvantages of each configuration - there is lots of good guidance out there on that. IF you opt for a RAID 1 level (whatever the hardware you decide on) then the answer to your question about storage amount becomes 1) how much do you decide you need with room for growth? then, 2) buy two disks of that size and fire them up as a RAID 1 config.
Another point - a RAID, even with a Level 1 config, is NOT a backup - even though there may be two identical copies on the two disks ... the two disks are there to allow you to recover "easily" should one of the disks die. plenty of events can cause you to lose all the data on the RAID (fire, theft, mistaken deletion of all the data, hardware failure, etc). A "backup" is an additional copy on ANOTHER device (either external disk (removable), or a totally separate RAID/NAS box on the network. "Way back when" I originally chose the ReadyNAS because it has USB ports on the back that allow you to REALLY easily back up the data (in whole or incrementally) to that external disk. That external disk can then be periodically moved and replaced, with the removed copy being placed somewhere safe (like a fireproof or filing box). I do this, and have something like four past copies that I rotate. If I ever get hit by ransom ware, I just give the finger to the ransomer, wipe my computer, reload OS and software from scratch, and recover my data (with little loss) from that disk in the safe .... EVEN IF the ransomware managed to propagate over the the NAS (which would be somewhat of a challenge, as it runs a totally different OS .... but never say never....)
I've been using and administering computers since 1971 .... and I am like one of the original data safety/backup wienies. Been there, experience that .... never again. It only took something like six years for my efforts to finally get through to my son (now in grad school) .... who now has a NAS with external backups
hope that helps...