[heptat]

Microsoft suggests 24x24: http://msdn.microsoft.com/en-us/library/cc849094(VS.85).aspx...

[cremno]

This site is about IE8. This one is about IE9 on Win7: http://msdn.microsoft.com/en-us/library/ie/gg491740%28v=vs.8...

The optimal sizes are the same as the author's but 24x24 px. is missing in the recommended size list.

The site links to a blog entry about Pinned Sites in Win8. You can re-use the 144x144 px. PNG for a pinned site.