Since steam is a gas, and gases will essentially expand to the size of the container, then 1 litre of water turned into gas water, will expand to whatever volume it is in.
In the open environment, the steam just expands off in all directions, however, as it cools, the water then condenses out of the air.
So if you have a hot shower with the bathroom door and everything shut, the room will fill with steam. When the door opens it will flow out. But you will also notice that there will be water droplets on the roof and the walls will feel wet, this is because the steam is condensing back into water.
Dustin has explained it nicely – the steam will expand to fill as much space as it has available if it is in a confined space.
Just for fun, I tried to calculate how much volume the steam produced from 1L of water could theoretically occupy at 100 degrees C and at atmospheric pressure if it behaves like an ideal gas. (If you haven’t learnt about gas laws yet then this won’t make much sense to you at the moment, but you can ask your teacher if you’re interested.) According to my calculations, the answer is about 1812 L! That is a lot of steam!
Is that the sort of answer you were after?
Let me know if you’d like me to explain how I calculated this.