You’ll need a rough idea of the size of your chimney in order to select the correct Chimney Sheep. We tried to keep it simple but chimneys vary in size from the size of a pipe to the size of a door. The Chimney Sheep needs to be an inch or two bigger than the opening in order to hold itself in place. The wool is compressive so it will squash to fit a range of shapes and sizes. And it can be trimmed down a little with sharp scissors if necessary. Have a look at the sizing info if you need help.