If it's a factory PowerTap wheel, I'd call them and ask.

It's also pretty straightforward to take another spoke out and measure it. Since you're going to be working on it anyway to put the new spoke in, this should be too much of an issue. If you're careful and have both pieces, you can piece the broken spoke together and measure it. If it's a J bend spoke, the length is measured from the inside of the bend (surface of the spoke adjacent to the head) to the very end.

The last option is to calculate it if you have or know the relevant parameters to plug into a spoke length calculator.