The advantage with Revit is that you can throw walls into a floor plan without too much commitment. Place the floor locked to the walls, then you can set up some dimensions and move things around so that the spaces start to show up sizes that are practical; insert furniture (which is to scale) and place measurements from furniture to walls and doors, place door swings and move them as you see where they might interfer with flow.
Many professionals will start with a survey or other constraint to the external envelope of the building size and work in from there as normally its not a totally open ended floor size.
once you play around with the floor plans and furniture and measurements you can start to work out the rooms that can actually be placed with practical outcomes.
After this is all set up in draft form having the capacity to move and change walls and all the components of the building you can start to look more into details if you want to add more precise rendering or colours (per Alfredo advise)
Design fluidity is where Revit shines as its so fluid if you ensure you lock floors and roofs / ceiling to the walls as it moves with your amendments. Once I have produced drafts showing full 2D and 3D layouts, I engage the client for design meeting and come away with lots of changes, but its not hard to make apparently complex amendments throughout the design process.
