I have taken figures from the 2011 Census to show the four towns in England whose populations lie closest above (and the four closest below) that of the Western Isles:
Farnworth (Greater Manchester) 26,939
Haverhill (Suffolk) 27,041
Melton Mowbray (Leicestershire) 27,158
Northfleet (Kent) 27,628
Western Isles 27,668
Ashington (Northumberland) 27,670
Cramlington (Northumberland) 27,682
Stratford-Upon-Avon (Warwickshire) 27,830
Peterlee (Durham) 27,871
The two Scottish urban areas with populations that are the closest above and below are:
Incidentally, the capacity of Cardiff FC's stadium is 27,815, and of Lord's Cricket Ground in London, 28,000.
The population of Uist, Berneray to Eriskay, (4,900) is close to that of Bridge of Allan.
I hope this helps envisage one aspect of the 130-mile long archipelago of Eilean Siar.
Sources: CnES Population Factfile, CityPopulation.de