The bigger the better! i bought the 'WONDERFUL' Martelli's Quilter's Edition Mat this last year. It is 36"x 24" It is also one of the only truly accurate mats around. it is yellow on one side and lavendar on the other side. It also doesn't buckle the way some of the others do.
Over the years I have bought many mats. If you are just starting to quilt, buy the biggest one you can afford, no matter what the brand. Quilt fabric is 44" wide, and folded selvege to selvage is 22"..........You don't want anything smaller than 22 or 24 inches wide.