If you go to the accuquilt site, you will see that they list the minimum mat size required for each die.
The quickest way to use your mat up is repetitive cuts in the same place. Use both sides of the mat, flip it end for end and try and position the die in different places on the mat. I just use 2 sizes of mats, biggest one, which you can use for any die, and a mid size one. You don't have to match the mat to the die. You can actually just buy the 10X24 size and use it for everything.