GameMastery flip-mats are 24 squares by 30 squares. The Incompetech graph paper generator lite can create graph paper to match: using a square size of 0.3125 inches will leave a half-inch margin on the PDF. Making a flip-mat-sized map using this method would require a canvas of 1200 by 1500 pixels.
The generator lite is also a way to quickly create 8x10 or 8x12 boards for variant chesses. An 8x10 board can fit on letter-sized paper with 0.9375-inch squares, leaving a half-inch margin. An 8x12 board can fit on letter-sized paper with 0.83-inch or smaller squares. Inkscape can easily checker these PDFs for a quick-and-dirty board.
I tend to prefer the square cross generator, usually. With a margin of 0.5 inches and 3 crosses per inch, it creates graph paper that is 22 cells by 30, which is only slightly narrower than a flip mat.
Hexagonal maps are useful, but the dimensions of hexagons are not as straightforward as other tessellations. Hexagons are usually defined by their side length, but the distance between parallel sides is more useful for my purposes. That's, very roughly, about 1.732 times the side length. Hexagons big enough to hold 25-mm miniatures should have sides of about 15 mm.