IMHO, (as a SU and LO user)
You should try to do as much as possible in SU, or anywhere besides LO, before sending content to LO for organizing and printing.
The lack of ability to draw to scale in LO or to accurately scale a scan as you wish to do is a significant limitation, and leads to the sorts of workarounds that Dave suggests. (which sounds like it will work)
In your case, you can import the scan to SU, scale it to size quite accurately, insert your other SU 3D items to the SU file, manipulate things accurately, and save some scenes to send to LO. This also allows you to show your sketch as the ground plane of a perspective 3D view (for example).