I agree with everyone about getting the biggest mat you can afford and accommodate in your sewing space. In addition to the mat I use for cutting, I use a 24" x 36" mat under my sewing machine so I can "fix" things without having to get up.
That said, there is a mathematical approach to picking mats and rulers. NO, DON'T RUN AWAY SCREAMING!!! Just joking.
Most quilt fabric is 42" to 44" wide; that makes it, off the bolt, folded over, 21" to 22" wide. If you want to cut a strip or, just square off the end, it's nice to have a mat that is at least, top to bottom, 22 inches. As for side-to-side, the board's width can affect how stable it is when cutting, stable as in not slipping on the table.
So, as many have already suggested, I think a large 24" x 36" mat is great for home and, if you can afford it, a 12" rotating mat for when you're squaring blocks. But, a 18" x 24" mat is great for taking to class (it also fits most pre-made quilting supply organizer bags). As for rulers, again because of fabric width, a 6" x 24" is invaluable!!! The next size down, 5" x 12"(?), is also very handy when piecing. Finally, a 12.5" square ruler is great for **squaring** blocks.
Hope this helps. At a recent class, I sat next to a newbie who was struggling with a 6" x 18" mat and 12" ruler. It was too painful watching her. Told her to use my mat & ruler -- she was a much happier camper after that!
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