Which disk sizes for Serveillance on DS420+ ?

[Indy]

I'm planning to add 2-3 cameras to my DS420+ which has no disks yet. Ideally HD quality 24/7 recording with 14 day retention. Can someone advise on the size of each disk, I also have 3TB of other data to backup to the NAS. Looking to go SHR or RAID5, thanks