Latest 44+ Fireplace Size Cm. If you have a fireplace in your home, you have the most popular amenity that people hope for when buying a new residence. Determining the size of an outdoor fireplace. My room size is 21ft x 25ft, with a 9ft ceiling height. Regency fireplace inserts are designed to fit inside up to 95% of existing gas insert sizing (opening front width x height x depth).