Not difficult:
Set up the font and size in tool options then open Edit -> Preferences
In the Tool Options section click on Save Tool Options Now
In the Input Devices section click on Save Input Device Settings Now
Ok that and close. Example screenshot https://i.imgur.com/hNZOQ0O.jpg
You do not absolutely need to untick the Save xxxx on exit in this case as shown in the screenshot.
Although not a bad idea to set Gimp up to start the same way on every start-up.
Set up the font and size in tool options then open Edit -> Preferences
In the Tool Options section click on Save Tool Options Now
In the Input Devices section click on Save Input Device Settings Now
Ok that and close. Example screenshot https://i.imgur.com/hNZOQ0O.jpg
You do not absolutely need to untick the Save xxxx on exit in this case as shown in the screenshot.
Although not a bad idea to set Gimp up to start the same way on every start-up.