That of the contouche itself would start out slightly longer than the petticoat at front edge, curve down to ground level at the sides and on to a short train the length of which should be determined by the fall of the dress. Look at pictures. Lastly, it is a good idea to sew loops of fabric or cord onto the inside of the waist seam. Cord fixed to different points of the skirt can be tied to them to hitch the dress up. This method was used by maids and anyone who couldn't afford to trip or wanted to protect the hem from dirt.
Some stomachers are pointy, some round, some have tabs.
The long pleat that makes up the front edge of the bodice could extend past the waist seam for up to 20 cm. If they had tabs that hooked down the centre front instead of a stomacher, they were called pet en l'air. Most of the contouches I've seen have pockets. That is, a bit of the skirt and petticoat side seam are open and neatened. In museums, you can see beautifully embroidered pockets that were fastened round the waist below the hoops.