Even though stem length isn't that critical within a fairly broad range...stem length based upon manufacture's specs tend to be frame size proportional.
For a very small frame 80-90mm will be a standard length. Most XXL frames will be the 110-120mm range for standard stem length.

But don't be overly focused on that if you need a slightly shorter horizontal reach but want the handlebar height of a larger frame. This is pretty common on non racing road bikes. Racers by contrast size their frames down to get a shorter head tube but in going to a smaller frame need to stretch out and why they need a longer stem for more horizontal reach. Recreational riders many times need exactly the opposite to get comfortable.