I use an Oxford Dormex size medium when the bike is in the garage. It’s breathable, so it doesn’t trap moisture, but keeps the dust off.
For trips, I bring an Oxford Umbratex size XL. It packs down super small and fits in the panniers even with a full liner bag in there. The XL is a perfect fit to cover the bike with the side cases on. The size large fits the bike well without the side cases on.