North Rona is 269 acres, so that must be right. But it is over 80 miles from the Flannan Isles - it's actually nearer to the Scottish mainland - and (unlike St Kilda) it's not a designated nature reserve.
Aquagility, it's not from the Flannan Isles (that's the 4th lighthouse), it's from the first one at the Butt of Lewis. I used Google Maps from Port Nis, zoomed out until I saw the dot of Rona (Ronaidh), then zoomed in on it. It even tells you on the map "Lies about 71km (44miles) NNE of Butt of Lewis".