I actually anticipate needing a somewhat larger home in FIRE. DH and I are not super social, and most of our social time is spent together (sometimes alone, and sometimes with others, though more often the former than the latter). However, we both require lots of solitary time, too. He's worked what most would consider insane hours most of our marriage. (We were tickled when he got an assignment that was only 50-55 hours most weeks! That lasted 3 years.) I anticipate having a "library"-- bedroom for me with no bed, bookcases, my favorite comfy chair, and which is painted in my favorite colors. I actually had this several moves ago, and it was heaven. And then DH will likely have a desk somewhere and a comfy chair of his own, and whatever else he wants. This likely could be combined with a guest room so we don't need another extra room, but it would then need to be a slightly larger room than if it were functioning solely as a guest room. Of we ever have a basement, it would likely work for one or both purposes, but we've never lived in a place where basements are a thing, and likely never will.
That said, I may already be RE, due to living overseas so long that it might be tough for me to find a job that pays enough to make it worthwhile. DH, OTOH, may follow roughly the same path my dad did. At nearly 74, dad is still working, though it's very part time as a consultant. Work, in some ways, is DH's hobby. I worry about what he'll retire *to*, and part of the solution for that may be that he always does some kind of work, if he can find a way to translate his future jobs into consulting or part time work.