If you are wanting the swing weight to be the same as the single length irons, you would need to know the head weight of the irons and compare it to the head weight of the hybrid. If the hybrid has the same head weight or can be adjusted to the head weight, then you could make it the same length as the irons. If not, then you will need to determine how long you need to make the hybrid to achieve the same swing weight as the irons. You must also take into consideration the shaft weight and grip weight. If the shaft is lighter, then that will effect the swing weight. I assume you would use the same grip. In short, it’s not necessary the hybrid be the same length, but depending on the head weight of the irons as compared to the hybrid, it could be. The most important thing, in my opinion, is to build the hybrid in the way that achieves the distance and trajectory you need for that club as it relates to the other clubs in the set.