Start by measuring your existing space and taking note of how the rooms feel. So you know that X room is 12' x 10', if you see similar dimensions you can go "oh, that's like our X room!"
As others have said, map things out to scale.
But when you get to the point of refining designs, I think it's super helpful to draw out a life size floorplan. It helped us a ton. More here: https://amodernmountainhome.com/2020/08/03/life-sized-floorplan/
Q
great idea to do a life-size version of the floorplan.
Q