2 3/4" casing is extremely common here and a store in town carries 5 1/4" baseboard for next to nothing. i've done my own house with it and it looks great. i also do alot of restoration work and do lots of floods. when we pull 4" baseboard and it was caulked to the wall we up size it to 5 1/4" because its the cheapest. i find every base has a matching case. put them up to each other in the store and decide from there and yes check to make sure the baseboard is thinner then the casing or it will look a little funny having to miter the base where it butts up to the casing.