Got the game yesterday. Works great, but the max resolution I can select is 1280x1024. Any way I can change this? I game on an HDTV so need to be able to run the game at 1080p.

You cannot go any higher than that resolution. 1920x1080 does not work. The game is too old and the highest resolution that the game supports is 1280x720, 1280x800 and 1280x1024 for 16:9, 16:10 and 5:4 respectively due to those resolutions being the maximum of that time.

Interesting!
Popenhance can go up to 1080p but unfortunately aspect ratio is not configurable:
http://www.tedtycoon.co.uk/wiki/PopEnhance#Patches

A question: is the populous config tool a custom built one by GoG? Would love to get more details on the tool itself.
I think it would be possible to hex edit the 1280x720 value and replace it with say 1920x1080.
If I get more details on the config tool I can do this myself and post a guide afterwards.