Complex sentence: According to the company that built the two parks, the Sandusky Park is 80,000 square feet but in December 2007 after the expansion, it will be 173,000 square feet and will be the biggest indoor waterpark in America.
Split into simpler sentences: According to the company that built the two parks, the Sandusky Park is 80,000 square feet. After an expansion in December 2007, it planned to be 173,000 square feet and will be the largest indoor waterpark in America.