How much disk space do you need on a server to run a full replication...

When running a full replication with PAReplicate, it is essential to have at least the same amount of disk space on the server as the disk size of the Primary Vault. This ensures that there is sufficient space to replicate all the data from the Primary Vault without any issues. The disk space should be equal to or larger than the total size of the data being replicated to accommodate the full backup1.

References:

CyberArk Docs: Install the Vault Backup Utility