If you are looking for a large map with two humans to fight against the AI, you can try mission 10 from
Co-op Knights I. That map is HUGE. Now, if you are not sure about what to make of it, you can watch the first 7 minutes of
my video replay to get an idea of it's too difficult or not. (I just explain the map in that time.)
If you are an advanced player, you can attempt Northern Islands I (also a two player map), which you can download from
this page. I also have a
video replay of this. This map is smaller than the above, but it has a lot more resources.
Should that version of Northern Islands I be too difficult (which is understandable), the map makers made an updated map.
Here's a link to download it.