Inside across is 11 inches each way, including the lip for the lid.
Measured across on the inside it is 10″. Revere actually called this a 10″
Measured diagonally it is 13″
Inside depth about 2 inches. The lid adds another 2 inches or so
These were made in both this size and a smaller size between around 1953-1956, along with the 12″ griddle, which will be another post. They are great for bacon, because they are square, but you can cook anything in them.
Does anyone have one of these?
Revere Ware USA 
I inherited these PR pots and pans when my mother passed in 2003! She had them for 50 years. They are WONDERFUL! Bought a set for my daughter many years ago. Both sets look awesome!!