How to set the spaces in a string format in Python 3

How can I set up the string format so that I could use a variable to ensure the length can change as needed? For example, lets say the length was 10 at first, then the input changes then length becomes 15. How would I get the format string to update?

    length =  0
    for i in self.rows:
        for j in i:
            if len(j) > length:
                length  = len(j)
    print('% length s')

Obviously the syntax above is wrong but I can't figure out how to get this to work.