A basic user entry is about 1 KB in size, as is a simple host entry with a certificate. The most important hardware feature to size properly is RAM. While all deployments are different, depending on the number of users and groups and the type of data stored, there is a rule of thumb to use to help determine how much RAM to use:
For 10,000 users and 100 groups, have at least 2GB of RAM and 1GB swap space.
For 100,000 users and 50,000 groups, have at least 16GB of RAM and 4GB of swap space.
For larger deployments, it is more effective to increase the RAM than to increase disk space because much of the data are stored in cache.
The underlying Directory Server instance used by the IdM server can be tuned to increase performance. For tuning information, see the Directory Server documentation