Ok so I think I figured out a way. I'm currently using Squarespace 7.1.
So I went into my edit setting on the map block.
Then I made sure "controls" are turned off in the design tab.
I hovered my mouse over the map and using my middle scroll I zoomed in.
Once I found the distance I wanted. I saved my page and it stay at that zoom level when i checked my page.
You can turn your controls back on if needed.
Let me know if that works for you!