I did a similar counter several years back with some 4" tall digits I just over drive the LEDs, for example if you have 4 digits and each segment is 20mA, pump each segment to 80mA. I also found it necessary to put a resistor to each of the 7 outputs to the LED segments rather than just a common resistor - else I found that a 1 was much brighter than anything else - and of course the 8 was quite dim.