Or, at least this worked for me.

Select the Launch Bastion icon in folder, right click & select properties.
In the "Target:" box, place the following:

"C:\GOG Games\Bastion\Bastion.exe" -lang en -windowed -x 1920 -y 1080

Change language & screen res` to whatever you want.

Shame this isn`t standard, I had to hard reset my pc several times before I figured this out, but ah well...