The string that is kind of thick and 52" is probably a dacron string. When a dacron string is off the bow, it tends to shrink. The skinny string is probably a fast flight string, and doesn't shrink much, but since it doesn't have many strands, it might have stretched when one strand broke. Putting this information together, I would guess that you want to order a 53" string, measured when it is fully stretched out.
Probably that, and the bow length of 56", and the other information about the bow that you posted, is enough for a bowstring maker to make you a new string. He might want to make the string slightly shorter or longer than my 53" guess, and I would go with whatever he recommends.
I wouldn't recommend shooting either of the strings that have broken strands.