I don't know a way that you can do that.

You can run  ^%GSIZE against a specific CACHE.DAT

I have 21 instances of Cache running on a single server.

There are a lot of pluses: simpler cpf, database snapshots, easier downtime negotiations

I would manually specify memory since you likely have very few simultaneous users.

Here is a link Sizing System Memory for Caché

If your session is text you might make a look at a tool like screen that is available for Unix based systems.