Sandy beaches, particularly wide bays open to the west, and good anchorages do not always go well together, and most of the beaches hereabouts are on the islands rather than the mainland. So what are you after? I am not sure how to guide you to what you might want but here is a rough classification of some nice beaches (I have ignored the not so nice beaches although they are mentioned on the anchorage pages):
Wide open classic beaches for rushing into the waves (and rushing back because it is so bloody cold):
Kiloran bay on Colonsay must be the best but the least likely for a quiet anchorage, followed by Calgary bay in the west of Mull. Other good ones include Balnahard bay on Colonsay which is quite a walk in for those on the island so it is not crowded, and Loch Buie on the south eastern corner of Mull which is very deserted, and the so-called Singing Sands on Eigg.