looking to buy 2006 foreman 500 es 4x4

Hello everyone, im interested in buying a 2006 HONDA foreman 500 es 4x4, time to upgrade to a bigger bike im currently riding a 2004 honda rancher 350 es 2x4 with 25'' itp mudlites,2500 warn winch i really like the electric shift feature and want the low gearing torque to put around and crawl through the mud and water. i woould like to know what size tires i can upgrade to on the stock foreman without losing too much power, i prefer to keep the motor stock and not add any engine mods except maybe a kn filter, i was thinking of going with the 26'' itp mud lites on itp c series wheels, is this too big or can i put 27'' tires or is it wise to keep the factory size of 25'' my size and weight may also be an issue im 6.0'' tall and wegh over 300lb will that severely effect the performance of this bike or does it have enough grunt to haul me around nicely with the bigger tires thanx, any advice would be appreciated this is cool site, very informative keep up the good work