Sea Princess was loaned to P&O many years ago and became Adonia the white Sister to Oceana, who was the original Ocean Princess after about 3 years she was returned to Princess. So she is the same size as Oceana. All balconies are the same size with exceptions for some mini-suites and Suites.
Well, I've not been on the Sea Princess, but on most Princess ships the cabins on Caribe deck are actually larger than the others. Half of the balcony is open and half is covered, so they're usually very popular.
All the balconies on sea princess are completely covered she is the same design as P&O's oceana.