OK I figured it out. I think this should be the default for your app but in the meantime using the following syntax will work. For anyone having game display problems on a retina mac with High Sierra, you can try the following:
Change directory (cd) to the folder with the game in it. For me, my user home folder is "dan", you'll need to edit this for your use:
cd "/Users/dan/Library/Application Support/itch/apps/FPV Freerider Recharged"
Then open the game using the OSX open command and --args parameter, with the app arguments as follows:open ./FPVFreerider_Recharged.app --args -screen-fullscreen 1 -screen-width 1920 -screen-height 1080
This launches the game properly for a normal experience, it's a workaround but it works!