Pellet grills and smokers actually pull a relatively small amount of power, compared to a common household tool anyway such as a drill. Therefore, you don't really need to worry about using an extension cord that's too long and causing a power/voltage drop.

However, what you do need to worry about is using an outdoor-rated extension cord. Don't use an extension cord that was designed to be used inside your home, they are not designed to deal with rain/water ingress and are a safety hazard.

Therefore whatever length of extension cord you use for your pellet grill/smoker make sure its suitable for outdoor use.