Scottish National Heritage have re-released a report from the 1980's about Scotland's 647 (yes, that many) beaches. It contains probably everything you ever needed to know (and a lot more) about these beaches. It's a 650Kb PDF file. Note that the data is some 25 years old, so due to erosion and the like some of it will be a bit out of date.
Some interesting facts in the report. The Western Isles (Outer Hebrides) has some 145 kilometres or 90 miles of beaches (though in their classification system, smaller beaches and coves aren't counted). Table 2.10 gives beaches with large bare sand areas; ten from out here are mentioned (though a bit confused as to why Luskentyre isn't on there, as well as Clachan Sands and a few others). Our very own Berneray is tabulated with 14 hectares of bare sand on the west beach.
The picture above is from Uig beach, Lewis. Plenty of sand there.