Size of Boxes for domestic travel

[Be_Kind]

Are Cardboard boxes allowed as checked in baggage for domestic travel? 
If yes, what size? 

[TheMiddleSeat · Accepted solution]

Yes, same rules as standard checked baggage apply. Length plus width plus height must be 62" or less and weight 50lbs or less or excess fees will apply. 

https://www.southwest.com/help/baggage/checked-baggage

--TheMiddleSeat