First enable "Analyze Errors In Solution" (right-click on the Resharper icon in the status bar).
Then right-click the solution node and select "Find Code issues". In the "Inspection results" tool window you can group by "Issue type" and look for "Type or type member is never used" to get all unused classes (and many more unused symbols).
A second option (after enabling "Analyze Errors In Solution") is to go to any unused class, hit Alt+Enter, select "Options for 'Type or type member ...' inspection" | Find all code issues of this type | Entire solution.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…