There are two basic types of gaskets used for pellet stove blowers. The first kind is the high temperature white lytherm gasket. These are more commonly used for exhaust blowers, but can be used for some convection blowers as well. Exhaust blower gaskets prevent smoke from going into the house. These gaskets withstand 1,000 degrees F and are used in the hottest stove areas. They usually fall apart when removed and are not reusable. These should always be replaced when doing the yearly blower cleaning. 6" round is standard for many exhaust blowers. Click here if you are looking for one.
White lytherm gaskets are also used on ash trap cleanout covers, burn pots, and exhaust flanges. Exhaust flanges are very important, just recently I worked on a Breckwell insert that did not have one. The smoke was leaking out and pushed into the room by the convection blower giviing the owner a headache! Ash trap cleanout cover gaskets seal the chambers behind the firebox. These covers are removed for cleaning and must be sealed for proper combustion. Usually, white lytherm gaskets are used, but sometimes they can be replaced with re-usable silicone gaskets. Burn pot or fire pot gaskets are usually white lytherm and prevent air leaks.
The second type of gasket used for pellet stove blowers is high temperature silicone gaskets. These are used for convection blowers to prevent noise and vibration. They are also used for ash traps and auger backing plates. These gaskets can handle 500 degrees F. They are used in areas of the stove that are not as hot. Silicone gaskets can be more easily reused. In some cases the auger backing plate silicone gasket can be reused. Check and see what kind of condition it is in. If it is too worn, just replace it when removing the auger for cleaning. This gasket prevents air from entering the hopper which also prevents hopper fires.
I always check the lytherm and silicone gaskets once a year during the annual cleaning to assure a safe and warm heating season.