Casselberry is a Sellers Housing Market, which means prices tend to be higher and homes sell faster. It is a city in Seminole County, Florida, United States. The population was 26,241 at the 2010 census.The city is part of the Orlando–Kissimmee–Sanford Metropolitan Statistical Area. According to the United States Census Bureau, the city has a total area of 7.1 square miles (18 km2), of which 6.7 square miles (17 km2) is land and 0.4 square miles (1.0 km2) (6.06%) is water. Its features over 30 lakes and ponds, the largest being Lake Howell. The Triplet Chain of Lakes, Lake Kathryn, and Lake Concord.