For both answers... yes you will need a longer device for 1e19 doping level. More or less around 10nm as you say (see graph in the tutorial).
To know if this is indeed enough you can plot the electrostatic potential across your device and verify, as the tutorials indicate, that your potential is fully screened, i.e. flat at the electrode boundaries. If not increase the length until you are satisfied.
Using a shorter device (well, it depends how much shorter) may give you qualitatively ok results, it's not possible to predict.
I suggest you to use a proper length.