Currently a 16GB key has more than plenty of space given only 3 cameras are in use... (4GB was plenty big enough when it was only 1 camera in use).
1.8GB is about what the full hour each camera will record before overwriting itself takes up... so 16GB would give you all 3 cameras using about 5.4GB, and leave you enough space to "save" another almost 2 more hours from all 3 cameras (in 10 minute clips as that's what it saves each time you 'save')
Thank you!
As it happens, I was placing an Amazon order and saw a 128GB stick on sale for like $18, so I got it. Sounds like it's WAY overkill, but oh well