I have the 24"x36" and the 18"x24" and use them both.....
Usually leave the smaller of the two out on the table all the time, ready at any moment when the urge strikes.
I also found a 6"x18" mat at a "trash & treasure" stand at a quilt show for $1.00, and I love it......so convenient to carry along to a class.
But as the other ladies mentioned, when cutting larger pieces of fabric, the larger the mat, the better.
MJ