the calculation for a bus bar depends on the current you want to draw. the no. of points u need and on the basis of that you calculate the current carrying capacity of copper on per sq.mm area(quality of copper plays a major role that is percentage of copper in that flat bar) and calculate accordingly. generally the width depends on the size of the nuts and bolts that you are using. and thickness varies again depending on the cross sectional area. But again a minimum thickness is required if the current drawn is less.