Is there anyway to adjust the resolution of the first game under OSX? I run it on my Macbook Retina and get a tiny game window on fullscreen, is there anyway to stretch that image?

I don't use the Mac version, but I guess it's running with Boxer. Boxer is based on DOSBox and can be configured by altering a certain file.

To adjust the settings for a single game, look inside the gamebox (ctrl-click and choose “Show Package Contents”) and open its DOSBox Preferences.conf file. This file contains performance and compatibility settings with tips for what each setting does.

To adjust the settings for all games, open <home folder>/Library/Preferences/Boxer/Shared Preferences.conf. The settings here affect every game but can be overridden by individual gameboxes.

Inside the .conf file look for an entry
output = surface
"surface" isn't scalable, so set it to another working option e.g.
output = overlay

(To toggle in-game between fullscreen and window, press Command-Key + F)

Ah thanks for your reply, but I just checked, it's actually running ScummVM. Should have been clearer about that in the original post.

I've done some Googling but I haven't had any luck in finding out how to adjust ScummVM's settings mid game. All the documentation is around the launcher, which seems to be disabled for the GOG release.

Thanks!