The map doesn't have to be square so long it is of (arbitrarily) 80% of the orinal (if smaller) or is 80% original (if larger). I.e. Feel free to remove smaller islands on the edges or complete cut off islands at the edges.
Also it... [should] be at a higher resolution than the sample
[the] map is a general guideline to be followed as closely as reasonable and as reasonable as possible.
For example there is no specified road network or indication of era so this could be anything from an alien planet with analagous features and domed cities to a frontier in the viking days to an established (romanesque) empire to a modern metropolis complete with high speed mag lev lines...
The focus can be as a sea chart or as a land map