Unsure on geo but it's one of the older ones.  An educated guess would be 130x30 (or even 130x35) it looks quite steep.  Here's a pic of it on my bike, it did feel NICE though. (I had a long GU)    Edit: If anybody knows the geo on these older ones then please educate me.  Second Edit: I was feeling helpful so went and measured, looks to me like 130x30