def get_img_size(n_fids):
    """Get guide image readout size from ``n_fids``.

    This is the core definition for the default rule that OR's (with fids) get
    6x6 and ER's (no fids) get 8x8. This might change in the future.

    Parameters
    ----------
    n_fids : int
        Number of fids in the catalog

    Returns
    -------
    int
        Guide star image readout size to be used in a catalog (6 or 8)
    """
    return 6 if n_fids > 0 else 8