ive heard 2inch is max, i saw one guy get 3inches, i personally have a 2.5incher, and no problems at idle while turning the bars, havent rode it yet, so it should be ok.
you can go 4", i have a 2001 sxr 700 and have a 2" riser and 2" pivot adapter. You need to route your spedo cable around air box, and let clamp around brake hose go or bend back.