I have one of the big folding cardboard ones (I used that when I used to cut out patterns for clothes) plus these self healing:
30 x 36 (this is a dritz brand and is junk so I don't use it for cutting-but it is good for if I need to protect the floor for pinning)
24 x 36 fiskars
18 x 24 fiskars (I have 2 but one is getting cut up)
12 x 18 fiskars
6 x 18 fiskars
the above two came in a value pack-it had the 2 mats plus a rotary cutter and a 6 x 24 ruler for $10. Bargain!