The busbar size will be determined by the total loads
connected to the bus.This will require a comprehensive
study of nature of loads plus the surroundings of place.
The general procedure will be such that the operating
parameters don't voilate the maximum safety limits of the
busbar sizes can not be calculated only from overall size.
there are various derating factors like arrangment of
busbar, nos of busbar, spacing, enclouser size, ventilation
in room, temp rise, skin effect, proximity effect etc.
which need to be consider while designing the busbar.
Usually busbar sizes selection depends mainly on two
factors. Which is current + temperature. The size which you
are going to select must carry the current you need and
must not create much heat which excess the limits.