Hi Brett,
Welcome to the forum!
The 3D model is created according to the dimensions that you used when drawing it in the first place on the plan. You see it in 3D as it actually would look - that is the purpose of the software. If you are having difficulties in fitting things in or moving through, then it is too small in the first place. If you are only concerned about being able to naviogate a model, rather than draw a 'real' building, then you would just have to draw it bigger.
However, something that may help, is to change the 'angle' of the camera view. When you select the Perspective view or create a new camera positon, you can change how wide an angle it is. The default is 60 degrees I think, which is about the average angle the human eye sees. If you make it 70 or more, it will give the impression of the room being bigger and be easier to move through. If you do this however bear in mind that it is giving you a false impression of how big it is.
Hope that helps,
Allan