I am running a Kopia repo which has grown to 73TB. The size itself seems not to be an issue, but depending on the number of snapshots, you might run into the issue I encountered recently, where the remote Kopia client is unable to list its snapshots due to an error invoking the GRPC API.
This actually means, that I am currently not able to restore any data on the remote client itself, since it can’t get to the snapshots manifests. The local Kopia client on the repo server doesn’t have this issue and is still able to list all the snapshots and thus also to retrieve data from them.
I did open an issue on Github for that and also tried to ping @jkowalski on slack, because I deem this a major issue for Kopia.