This is a universal gasket used by many manufacturers to help seat the combustor into place. It will swell when subjected to the high temperatures in the combustor chamber. Usually it will crumble and fall apart when the combustor is removed, so it should always be replaced whenever a catalytic combustor is removed for inspection or replacement.
If you are not sure what your stove may require, consult your wood stove dealer, or review the stove owner's manual for information.
To calculate the amount needed, add the total length of all 4 sides of a rectangular combustor, and round up to the next highest foot. If it's a round combustor, simply multiply the diameter by 3.14 and round up to the next highest foot.
Note that some wood stove manufacturers may require the use of a special gasket, and this will be supplied with the replacement combustor when needed. In this case, our descriptions will state if a gasket is included with a particular combustor.