As your surmise...
Scan it in as a PNG or JPG.
Take that image file and import it, as an Image [choose under Options].
Make a group of the selected image, so what you are about to do only affects that image.
Edit that group.
Use the tapemeasure tool and pick two points on the image which are a known distance apart - e.g. a wall length.
The measure-box will show some arbitrary length.
Immediately just type in the desired length + enter, when prompted choose to re-scale...
The image is now resized so that at is sized properly [or at least as good as your hand-drawing!]
Exit the edit.
Select the group and use the context--menu to lock it - that way you can't accidentally move it as you add walls etc.
Now start over-drawing it.
Use the Rectangle Tool to make room shapes, walls and openings etc.
Don't worry initially about extra lines and faces that you don't need later - you can erase/delete those later...
If you have any non-orthogonal edges draw those lines in manually.
When you have most of the form down in 2d, then you can delete the unwanted edges/faces.
Now PushPull your flat 2d faces up in 3d to make walls etc...
This is a very simplified - but you should get the idea...