You want to do something like, right?!
https://dryuc24b85zbr.cloudfront.net/te … 0519669808

This is usually done this way:
1. You create one big grid texture.
2. Using MapTiler or another similar service, you divide a large texture into tiles.
3. Import the tiles into the project.
4. Using this example, display your overlay.
https://infinity-code.com/atlas/online- … ample.html

How do I tell the size in miles for each tile? I'm using 512x512 when I create the textures.

The easiest way to set the grid texture, into the OnlineMapsTile.overlayBackTexture field for all tiles:
http://infinity-code.com/doxygen/online … 6e065be26c