all outside cabins on decks eight and lower are of the same size, so you would have to upgrade at least to a Cat. D cabin to get a slightly larger room. As for the location, the higher in the ship the more expensive the cabin is. However, cabins on lower decks are subject to less movement.
Personally I would stay with the cabin you have booked. All cabins onboard "Sovereign of the Seas" are pretty small, but it's okay for a short cruise and even the "larger" cabins are not larger than the standard cabins on ships operating longer cruises.
Enjoy your cruise!