Is there a standard length on the PVC furnace pipes coming outside the exterior?
The pipes have to be long enough to clear any overhang or roof.

This is to ensure that the exhaust gases don't collect under the roof. This could be a fire hazard (if the gases are hot), though this isn't that likely, or a health hazard (there'll be high concentrations of CO2, or even CO, in the gases. If these re-entered the house it would be a definite health hazard.

There could also be a lot of moisture in the gases and you don't want that condensing on the overhang either.