I think the most important thing to realize is that you don't have to make this decision NOW. In fact, it's probably best that you don't make decisions just before a big life-changing event; you don't want to make decisions out of desperation, fear, or without full information.
The house you describe will absolutely serve for the time being: You can have a room for yourself and for the baby and still have ample room for guests. The baby won't need a bathroom for quite some time. If and when you decide to have another child, they can share a room at least as small children. If that second child never materializes, the house you have will continue to serve -- and, depending upon your mindset, you might be fine with two children in your current house. However, as the mother of two teens, I am glad that my girls each have their own room to which to retreat and a shared bath of their own. When you reach the point that you determine more space is a NEED, you can move then -- hopefully you'll be earning more money by that point and you'll be more sure of your actual needs (i.e., you'll know that the second child didn't turn out to be twins!). Also, life tends to happen, and work or other family needs may force you to move before you actually need more space, and you might take advantage of that "push" to upsize. The moral: Just wait. You're fine for now and can always make this change later.
As for guests, when I was a kid, it was always simple: Grandparents are coming. Kid #1, clean your room -- they're sleeping in it. Kid #2 -- clean your room -- Kid #1 is sleeping on your floor. No problem. In my house now, we have a sofa in the office, and we have air mattresses. My husband and I are in the process of putting together our thoughts for a home we plan to build, and we have a unique thought for guests: We want to buy a teardrop camping trailer anyway. It'll contain a queen mattress, an air conditioner, and a heated blanket. If we have enough guests that they can't fit in our bedrooms, someone will sleep in the teardrop.