What size spoke gos in a 700c rim?

There's no set answer to that. 700C AKA 28" AKA 622 mm ETRTO tell you the diameter where the tire will fit, but it doesn't say anything about where the spokes will go.

Things that influence spoke length:

• rims, high or low profile
• hubs, high or low flange
• hubs, hub flange spacing. Rear wheels will usually need two different lengths, as they are often assymetrical, as are disc braked fronts.
• lace pattern, number of times a spoke cross another spoke, more crosses need longer spokes
• Spoke thickness. Thinner spokes stretch more, so they need to be a tad shorter.
• type of nipple. The nipple is what attaches the spoke to the rim, and they're available in 3 different lengths.

If you just need to replace a single one, remove another spoke (from the same side of the wheel). Measure from inside of bend to the tip(in mm) and you'll know what to get.

If you want to build a wheel, well better do your homework first. I'll post a link below.