How to enlarge my savefile?

You can enlarge the savefile using the "Fatdog Savefile Tool" found in the Control Panel, under the "Install" tab ("System" tab on older Fatdogs).

However, you may only enlarge the savefile when the savefile is not being used.

To do that reboot with the savefile=none boot parameter supplied to the boot command line for the session. Then you can use the savefile resize tool.
Note that the initial size of a btrfs formatted savefile must be at least 256 MB, otherwise the btrfs utility won't be able to enlarge it.