Do brake hoods come in different sizes? I have a Ribble Sportive Carbon road bike.  The brake hoods are much smaller than most other bikes whihc makes them much less comfortable than other bikes (Scott, Specialised etc.)  Can I just replace the hoods to make them more comfortable or put larger hoods over the top of my existing hoods?
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