Welcome to flyertalk. The best source would be from each airline's website. Anything else would probably quickly go out of date.

From the American Airlines Luggage Page:

The size limitation of your luggage is calculated by adding the total outside dimensions of each bag, that is, length + width + height.


Normal size bags have to be 65 inches or under for the sum of the three dimensions. If it's over 65 inches and less than 126 inches it would be oversize, and subject to a $150 extra cost in the US.

So yes, if your package was 10 feet long and three inches high and wide, it would still be able to be checked for a fee.