I have a Trek 7500FX with Bontrager race lite 700x32 tires. I have broken spokes, I no longer have the book what size spokes do I need?
"The correct length" can be found by measuring an existing spoke. If your broken spokes have failed at the hub, simply measure from the thread end to the inside of elbow/turn and buy that measurement. Otherwise take off a known-good spoke, measure, and reinstall.
I've occasionally had to shorten spokes by 1-3 mm because they poke through the nipple into the tube. 10 seconds with a dremel or a couple of minutes with a hand file are all it takes. Whereas you can't lengthen a spoke, so if in doubt go for a 1mm longer spoke.
Depending on how many spokes have broken, you might find the adjacent surviving spokes are overtensioned now, so get a couple spares, and a decent spoke tool if you don't already have one.
EDIT: Moz makes an excellent point if your wheel is a low spoke count (like in the picture) If you're not feeling confident, it may be safer to take the wheel to a shop for spoke replacement and true and tension check.