B.,
I agree with you completely and I also see your husband side as well. Men when it comes to blended families tend to over compensate, it is logical for the 10 year old to have the bigger room if she lived there full-time, but what you have to explain to your husband is that for Alexis this is her second room. She does not need as much space because she will have less stuff. The room size should not be distributed based on age, but need. As your daughter gets bigger so will her things (toys, clothes, etc.,) and you will need more space. With that being said I think this is something that you need to discuss now instead of later. Once Alexis is in that room trying to get her to switch will be more dramatic then just taking care of it now. I suggest you have a discussion with your husband about it and remember it is not what you say, but how you say it. And allow Alexis to have a big part in decorating her new space so that no matter which room she gets she feels like it is her space, that is all kids want anyway really.
Good luck, been there, done that!