The family and I are getting ready do move in to a new house. I just found out i am pregnant. My husband and i?

of course will have the master bedroom but i don't know how to determine between my children who gets what room. Theres three other rooms 2 that are same in size and the other one is larger than the both. I currently have a 6 yr old stepson and an almost 2 yr old daughter. I wanted to save the bigger room for the newborn. Is this fair? Who should get the bigger room? Most would say the oldest but i still dont think thats necessarily fair either. I don't know what to do.