I have the GBC X-rexs on mine... size 22x11x9... they just barely fit with the stock heel guards... so if your guards are any further back then the stock ones then don't get too big a tire... and for a $160(ebay) they were a awesome deal... great traction.. and still can slide if need be...
2004 YELLOW TRX400EX
15/39 Sprockets, 2005 Lower End, 22X11x9 GBC X-Rex,
23X8X10 Interco Swamp Lites, 250X Clutch Cable,
Aftermarket Swingarm Skid Plate,
DG Headlight Guard, Moose Racing/DynoJet Jet Kit,
Moose Racing Bars, Moose Racing Aluminum Hand Guards,
Moose Racing Comp Handguard Protectors, UNI Air Filter,
Moose Racing Muffler Packing, Pro Armor Tether Kill Switch,
Sintered Brake Pads, Team Alba Pro Elite Nerf Bars,
White Brothers R4 Slip on, White Bros Air Box Vents,