Here are Southwest's guidelines for carry on bags:
# Airline passengers may now carry only one bag plus one smaller, personal-type item onboard the airplane.
# Personal-type items include purses, briefcases, cameras, food containers, or laptops (case included).
# Southwest Airlines limits carryon bag dimensions to 10x16x24 inches.
And for "regular" luggage:
# 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.
The last few times that I've travelled, I've noticed a crackdown on the weight of the luggage. I've seen quite a few people re-packing suitcases at check in desk. Hope this helps!