Half-Double Crochet Stitch
When I make the plastic mats, I start with 75 single stitches for the bottom row, then use the half-double crochet stitch for the rest of the mat.
This will make the mat approximately 2-1/2 feet wide or so, and I usually make them about 5-1/2 feet to 6 feet long.
You can make your mats as long or wide as you like, using any stitch you like. This is how I make mine.