def find_allowed_size(nx_size):
    """
    Finds the next largest "allowed size" for the Fried Phase Screen method
    
    Parameters:
        nx_size (int): Requested size
    
    Returns:
        int: Next allowed size
    """
    n = 0
    while (2 ** n + 1) < nx_size:
        n += 1

    nx_size = 2 ** n + 1
    return nx_size