Unfortunately there is no formula. What you need depends highly on what you are going to monitor.
Logging in Icinga 2 can be configured. So if you plan to start the debuglog from time to time, plan for more. For Icinga 2 alone a small log device might be enough. Depending on your hardware costs I’d go for 10GB, just to be sure. In fact you will need far less than 1GB (for Icinga 2 alone, without debuglog), but since storage is cheap I’d take the safe way.
If you want to have a logmanagement setup with e.g. Elastic Stack, you should stick to their best practices. While it’s totally possible to run Elastic Stack on only one host, you should definitely go for at least 3 nodes. How much space you need is totally dependent on what you’re going to log. Since logs come in all styles in syntaxes you might need from a few GB to hundreds. Also take into account that you will want redundancy (at least double the space you calculated) and that it’s totally dependent on how long you want to keep your logs.
For smaller setups running Elastic Stack on the same host you’re using for your Icinga 2 masters is totally ok in my experience.
You’re mentioning you want to monitor workstations. If you mean “logfile monitoring” by “monitoring” than that’s possible. But keep in mind that Icinga 2 was not made to deal with hosts that can or can not be online. And of course be double careful not to break any privacy laws!
In fact size calculation for Elastic Stack or Graylog means starting small and adding storage or nodes as you need them.