I am installing a second circuit breaker panel in my home. I will be supplying the service from a 100 amp breaker in my old box to the main breaker in the new box. What gauge wire do I need for this supply?

Is there a main breaker in the new box? If not, you need to determine what the total load on the new breaker box will be. Then you need to determine how big the main breaker will be in the new box. How many Amps will you need? Ever done this before?

Yes, the new box has a 100 Amp main breaker in it. I will be fed from a 100 amp breaker inserted in the old box. I will keep all the 220 circuits and large draw items in the old box.

You did not answer ma's question about what will be the new load.

To make it more complicated, need to know the connected load of the existing service to be sure it can handle additional load.