What size cone wrench am I likely to need for a Trek 1.5 2011 Compact rear hub? Unfortunately the manufacturer's website, and google don't appear to hold this information. The only thing I've found is that the hubs are aluminium which doesn't really help.

Is there a common size of wrench? If I get the size wrong I can always send it back but I'm guessing people who deal with road bikes a lot will know that there's a common set of sizes.

Any suggestions? :)
I had a 2008 version of the 1.5 bike and the rear wheel took a 15mm cone wrench. 

It's possible that Trek changed this, but I think 15mm is a good bet.

I agree with @alex; anything other than 13, 15, or 17 would be considered uncommon.