swallows nesting in an old mill

We want to let swallows in to the front of our old mill as usual. However, last year we had a whole flock of pigeons taking over the space which we want to prevent happening this year.

We were thinking of closing the shutters where they normally fly in, but creating a hole for them that would potentially be too small for a pigeon. Does anyone know what sized aperture would be the best, and would the baby swallows be able to fly back through after they have fledged? Are swift boxes good for swallows?

We need to act fairly quickly as they will be arriving in just over a months time so any advice will be much appreciated

Many Thanks,

Susan Phipps