A map sampler was one of my first antique purchases in 1986 and I have loved them ever since. My map of England was stitched in the year that Wellington won the Battle of Waterloo! This sampler was inspired by my antique map collection and is worked in cross stitch with backstitch detail. There are a few fractional stitches so you may find it easier to work this on linen rather than Aida.
To work out the design size (or the stitched area) you need to divide the stitch count by the number of stitches to the inch (2.5cm) on the fabric. You then add enough fabric to this measurement to allow for framing etc. If a stitch count is not shown on any design, you will need to count across the design and down the design, which will give you the stitch count.
Example: 140 stitches divided by 14 (Aida) = 10 inches
If working on evenweave fabric, remember that you generally work over two fabric threads, thus 28-count linen is also 14-count when doing the sum. I suggest adding 5 inches to the design size on a project to make sure you have an adequate margin for mounting and framing. Obviously you can adapt this amount when working a small card of lid top.