The 2008 and later Kawasaki KLR 650 has a rear tire size of:
130/80-17. A good tube size for the rear tire is 5.10-17
and a front tire size of:
90/90-21. A good tube size for the front tire is 3.00-21
Not sure if pre 2008 KLR 650's used a different tire size. If you would like to know more about these numbers, and tire sizes in general, than view this forum post:
http://www.thesmallengineshop.net/smallengineforum/index.php?topic=192.msg349#msg349
