I upgraded the Mint server to Mint 18. This broke x2go in a small way. The x2go xserver has version 1.2 of xrandr, but Mint 18 requires at least version 1.3, so the screen sizing fails.
However, the xrandr command is version 1.5, so when you first connect with x2go and get a small 800×600 pixel screen open a terminal and type:
xrandr -s (width)x(height) in pixels, for example, my screen is 1680×1050, so to get the display to use the whole screen I type “xrandr -s 1680×1050”.
This is only necessary temporarily until I can figure out how to resolve an issue with xagent version and GPG keys.