I think 19-20 inchers are about it unless you throw some spacers at it to clear the front frame corner.
scroll about 1/2 way down for pics from GX-150 with bigger tires.
Metal's spiderbox threads have a ton of great info in them, worth the time to read through. pretty sure the topic has been asked and answered several times now.
http://buggymasters.com/forum/showthread.php?t=1159