I need new rear rotors and I have noticed there are 2 different diameter rotors available. One is a 234mm and the other is a 250mm. But, it also appears there is only 1 caliper available for the rear. Mine are 234mm and was curious if I could get the 250mm ones?

Yes you could get the 250mm rotors. But there is a catch. You need 89-94 Maxima Calipers for those rotors. The Maxima calipers are a direct replacement. I don't know why there are two different rotors listed cause I have yet to run across a USA NX that has 250mm rotors stock.
The Maxima calipers and rotors are the rear brake upgrade. The 250mm NX rotors are the identical size rotor, just a fraction of a mm thinner. I think you need an Altima MC too. That I am not sure of.