from the southwest website
http://www.southwest.com/travel_center/baggage.html
Baggage Allowance: For each ticketed Customer, Southwest allows three (3) checked pieces with size limitations for each individual piece.
Excess Baggage: Excess baggage will be charged at the rate of $50.00 for the first (1st) through the ninth (9th) extra bag and $110.00 for each piece checked thereafter.
Weight and Size Allowance: Maximum weight is 50 pounds and maximum size is 62 inches (length + width + height) per checked piece of luggage. Effective March 1, 2005, overweight items from 51 to 70 pounds will be accepted for a charge of $25.00 per item. Items weighing from 71 to 100 pounds and oversized items in excess of 62 inches but not more than 80 inches (i.e., surfboards, bicycles, vaulting poles) will be accepted for a charge of $50.00 per item. Any item weighing more than 100 pounds must be shipped as Air Cargo. However, Customers cannot use SWA Cargo unless classified as a Known Shipper as defined by the FAA or FAA approved Indirect Air Carriers (IAC). For all Cargo inquires please call the Cargo Sales and Service Center at 1-800-533-1222.