800cc is indeed the maximum rated volume that I heard of.
It basically means that, at that volume, they're fully filled with no overfill or underfill.
Regular saline implants are safe to overfill by about 20%, which explains why lots of girls have 960cc.
Expanders can be overfilled to about 500% of their size, so they can reach 4000cc, but their starting, rated volume is 800cc.
So, in the us, the maximum 'rated' size is 800cc, but they can be overfilled to 2500cc.
The problem with that is: Expanders aren't exactly known to be very safe. They're not meant for permanent use. Their primary use is to expand tissue (hence they're called tissue expanders). Designed to grow skin for burn victims or breast cancer survivors. So: stretch the skin and remove the implant, and possibly use the skin elsewhere, or replace with a more permanent implant.
This permanent implant will have the problem that it has to be a regular saline implant, which cannot go over 960cc. If you stretch the skin too far, and then having small implants will look all weird. So you need to keep the unsafe expanders in. Which will likely become a problem.
Outside the US, it's a different story, there are larger expanders available, but difficult to track down. But there's also larger regular implants available that you can use to replace the expander with. You just can't get them in the USA.