I cannot get Imperialism II to load in a large game window.

From their game forum: "To answer your question, I'm on Windows 10 in 64 bits and to display the game in full screen you have to go to the exe "Imperialism.exe" and in the compatibility tab check "run with a screen resolution 640x480" ."

This did not solve the issue, it worked fine until GOG updated

Do you mean GOG updated the game?

gogdb doesn't report an update to the game since 2018. Are you referring to the recent GOG Galaxy update?

Sounds to me like fullscreen does not work.
Try Alt+Enter

Another possibility is that the game is running full screen, but their graphics card configuration is centring the image instead of scaling it. We'll never know if they don't come back with more info.