Do what we did. Take the dimensions and tape them out on your floor. Even so, the room will feel smaller when you get there.
We used the Family Bedroom when we traveled to Glacier Park - it was significantly less expensive to do this than to get two roomettes (by almost the price of a roomette). In fact, I leave in two days for Chicago, and because I had a voucher, I only had to pay an extra $60 to get the Family Bedroom again - we won't be sleeping, but having some private space will be nice as well as having the meals included.
The space is long and narrow and when the bunks are down there isn't much room to navigate. My children are old enough that the next we would probably get two roomettes if the price.
Advantages: Everyone together in the same room, windows on both sides of the train, lower level so very little traffic
Improvements: The door will not stay open unless you have a doorstop (our room was very hot), the windows are small, you're close to the trucks and there can be some rail noise.
Bottom Line - you'll have an enjoyable time and it will be whatever you make of it.