Hi confrère. 
I work wit a basic A3 sheet with 300 dpi.
Then I know that 118 px are 10 mm.
So I use the rectangle tool. For exemple if I need 30mm I put in the box 11.8*30 and I lock the proportion. Maybe the picutre is more explicite.
Then even if you import some pdf (from autocad) you rasterize at 300dpi for A3 size the dimentions will be correctes.
Finally I colorize my sketch and I add some details too, without go back in Autocad.
And you export always as A3, 300 dpi.
This is my workflow.
ps.
I see your profile. Great work! Your really good.
FĂ©licitation.
