Do brake hoods come in different sizes?
yep they come in different sized based on what brand the gears are, and what year or model.

Campagnolo, Sram and Shimano the 3 main manufacturer's. 

Looking at the stock Ribble sportive bike i'm guessing you will need these: 

http://www.wiggle.co.uk/campagnolo-ergopower-replacement-hoods/

or these

http://www.ebay.co.uk/itm/One-23-Replacement-Brake-Hood-Covers-for-Campagnolo-11-speed-WHITE-NEW-/280905184450?pt=UK_sportsleisure_cycling_bikeparts_SR&amp;hash=item416740f4c2 

Campagnolo 10 speed, or modern 10 speed replacement hoods