I would have originally assumed 2.0 millimeter, but it seems that earlier models of this bike used 1.8mm straight spokes too, so it's difficult to say.

Without a caliper, you can use a more tedious method:

Cut a 1"x1/4" strip of paper (doesn't have to be exact)
coil it around the spoke, then unravel it so that the end coiled under just barely meets the strip coming around (fully encircling the spoke) and mark it with a sharpened pencil.
Take that sheet and measure the circumference of the spoke by measuring from the edge of the paper to the line.

If it's almost exactly 7mm, then you have a 2.0mm spoke. If it's under 6mm, you have a 1.8mm spoke.

It's not terribly accurate, so it may be worth repeating a couple of times, but the difference is significant enough that with a sharp pencil and a decent ruler you should be able to figure it out.