You probably want to make them as long as practical to fit in the limited space. I've got ~600mm runner on my A15 but haven't gotten around to dyno or anything yet. I was aiming to have my VE peak around 6000rpm.
You can use this
http://www.bgsoflex.com/intakeln.html (as I did) to help estimate wave tuning, it's probably within 10-20% and would stay about the same under boost. The only trick is that runner length is from valve face to just outside the port inlet but it's approximate anyway.
Inertial boosting usually means the longer the better until friction starts slowing down air flow. (You wont get anything that long under the bonnet anyway.)
For the Plenum there's no easy way to estimate, but in general a smaller one lowers the torque peak (in rpm and absolute) and spreads it out. The bigger it gets the more it starts to behave like ITB's open to the atmosphere.