Hamish Haswell-Smith is still the undisputed expert on Scottish islands. While comparing the commonly held top 10 largest islands in Britain against our list from Ordnance Survey I was surprised to find a few points of difference. Casting around for some official figures I was astonished to find that there didn't seem to be any. Indeed, both Wikipedia and RJ Berry in the New Naturalists' Islands quote Haswell-Smith's calculations. For his results he used a planimeter to measure "the total surface area lying above sea level at Mean High Water Springs." Out of curiosity I thought I might check his results using some "digitalised map data" that he predicted would be available in the future. I used Daft Logic's online map calculator tool and after some initial excitement spent way too long plotting points around the perimeter of Jura, mimicking the planimeter but with less accuracy. Anyway, the result of my fruitless evening suggest no reason to quibble with Hamish.
My calculation for Jura is 36,359.38 hectares, Hamish's is 36,692 hectares.