I never was a big fan of the "Mystara" style of mapping. But in the 80s Mystara was just as popular as Greyhawk if not more. I did one large map using the style. It wasn't anything related to any of my campaigns at the time I drew it just because. I believe I was thinking of 30 or 60 mile hexes here.
The circular sea to the west was meant to be a meteor strike or a big magical explosion. In the northeast is a complex coastline that is meant to be fjords a viking home. One of the large southern islands was supposed to be the X1 Island of Dread. The center area of the southern continent suffers form rainshadow, very arid and has lots of deserts.
If it inspires you feel free to use it for your own game. If you want to clean it up a bit try Hexographer which is one of the best mapping software out there for the Mystara style.