As is it uploaded it’s 1920 px wide. To get a nice sharp “in hand” engrave I’d try to stay at 300 dpi or better so 1920/300 = 6” wide. If this is intended as a wall hanging you can get away with 200 dpi, or about 9.5” wide.
If it were me and I wanted it larger I’d try to get the initial resolution higher, if you want the map 15” wide, I’d try to get the source image at 4500 px wide. Make sense?
Then I’d proceed with converting it to greyscale, adjusting curves to get good contrast, and then use 3d engrave/vary power. Using that technique worked really well for me. Hang on, I probably posted the project i’m thinking of.
Edit:
Ok apparently I never posted this one. Fixing that.
Edit edit:
Here you go -