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Until the maps in Skwyre are more configurable, here are two FM maps you can use as templates. There's two here, one is a 400 foot x 400 battlemap, and the other is 200 foot x 200 foot. Each have 10ft grids, which correspond to the 'major' grid lines in Skwyre's map page.
When exporting as an image, export the 400 foot map at 4000 pixels x 4000 pixels, and the 200 foot map at 2000 pixels x 2000 pixels.