I'm on OSX. I've looked around in the package contents but I can't find a config file that I might be able to edit.

Just wondered if it's possible and how to go about setting it up

I found the answer on a more general WINE forum.
Go to the installed files. Right click on the package and select 'Show package contents'.
run the Wineskin file.
There's a load of options in there.
I Opened the screen options and selected "Virtual Desktop at:"
I then played around with the resolution options until I was happy.

The test run from wineskin didn't seem to run, but the app runs fine from inside GOG.