What size cone wrench am I likely to need for a Trek 1.5 2011 Compact rear hub?
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.