You can keep your existing spacer and bolt but that is assuming that it was correctly setup to begin with (and you are using the original wheels). The technical service instructions have a chart to determine the correct spacer and washer arrangement but it doesn't list the actual sizes.
http://bike.shimano.com/media/techdo...9830749996.pdf
I'd guess that 15mm is the long spacer. The medium spacer looks like it's length is about the same as its diameter, and I don't think the diameter would be 15mm.