The collector of a bipolar transistor can act as a constant current source. With a resistor in the emitter circuit, the current will be proportional to the voltage across the base junction - emitter resistor.
The size of the display is not significant, it's how much current each display segment requires.
dear carl the idea of collector as a constant current source is not driving the segment properly it requires atleast 30 milli amp cirrent with voltage variation of 7.5 to8 volts ....
more over when i connect all the segments of this active low 7 segment the is a tremendous voltage drop in the output of the segment resulting in the increase of voltage across resistor and heating them..........