I am not sure which kit you have, but I just made one (artisan) and length is dependent on what type of cigar you like. For example, I made my tubes 5.5 inches long total since I know my recipient smokes robusto size. When you press together, the center ring also adds some length.
If you got the same holder, it also requires a 24.1mm (61/64)Forstner bit to bore out for the tube set.