Corso, as i will be mainly using mine in summer on a sandy beach i will just have the serge material stitched inside the canvas and a wool blanket inside that,probs sleeping in lightweight thermal undies,nothing else with it or under,but i will be sleeping near to a fire, years ago i used to own a proper ww2 one and i slept out,in a forest in winter i had two blankets inside and a ww2 ground sheet underneath and was dressed in ww2 wool battledress clothes(was at reenactment event)and was okay temp wise,though if i was doing winter stuff now with it id have a foam roll matt or my alpkit airic slim underneath,and again two blankets and thremals on.The heavy canvas really works to cut out wind and damp most seem to come water proof coated,although in time id probably treat the base with something like barbour thornproof dressing.
All really depends when/and on what type of ground you will be using it really,i aim to have mine around 6'6''(and to have the envelope shape at the top to store my dry clothes and act as a pillow) in length and plenty wide enough to move about in width wise so i could add more insulation or take away depending on the conditions.