For the forward modeling of the electric potentials (EHG), you just need one 3D position for each sensor (the center of the electrode).
For the magnetic sensors (MHG), you also need the orientation of the coil to know along which axis the field is measured, and in which direction.
If the sensors are gradiometers, you need to know the position of the two coils.
For a more accurate modeling, Brainstorm uses the geometry of the sensor to estimate the leadfield at different points in space around the center of the sensor:
http://neuroimage.usc.edu/brainstorm/Tutorials/ChannelFile?highlight=(integration)#Edit_the_channel_file
This is optional but recommended.