My favorite rulers are the Olfa frosted rulers -- they don't slip. You can usually find the 6"x24" in quilt/fabric stores, but the JoAnn website has several of the other sizes. The ones I use regularly are 6x24, 6x12, and 12.5" square.
I'd buy the largest cutting mat you have room for. I use the markings as general guidelines only, never for accurate cutting. I've reached the point where if I don't actually need something specific at JoAnn's or Beverly's but I have a coupon, I buy rotary blades. You never want to run out of blades!