Dear user,
That can be done, although you will want to set the scale with which you want to draw before doing anything else. To set the scale, open a drawing in SmartDraw and go to the page tab and choose "Scale > Custom". Set the scale you wish to work in and click OK. Drawing to scale is outlined here:
http://www.smartdraw.com/resources/how-to/Getting-Started-with-Forms#/resources/how-to/Why-Scale-Matters
Initially, I would recommend a scale of "One inch on the screen equals 2 inches". That way the 12" object will be 6" inches on the work area (assuming you are printing to letter size or A4 paper).
From there you would use the shape or line tools on the Home tab to draw the diagram out. In addition to that, note that we are not 3-D so the depth of the object will need to be a separate diagram on the work area. So you will need to draw a front and side view.
If you have further questions, let us know.
Regards,
SmartDraw Support