I’m 6′ 0″ tall and have never wished for more leg room through a longer chassis or extended porch. It’s really a personal preference but figured I’d throw out my experience as a data point for you to consider…
Just about any chassis that a 4 cycle motor can be mounted to can be fast. The fastest guys at my club run all sorts of different chassis’. One runs a DR 4 cycle, another a CRG 4 cycle, another a 30mm CRG, another a 32mm Birel, another a 32mm Intrepid, and the list goes on and on. The point being these guys are fast on old/new chassis of varying brands with varying tubing sizes. My recommendation is to find a kart that is supported locally and if there isn’t one than get some recommendations from the guys at your local club.
I will put in my two cents if you want to go brand new. To keep flexibility I would recommend a 30mm, 30/32, or a 32mm chassis (you never know if you will want to switch to TaG or at some point you may decide to sell the chassis and these appeal to a larger crowd). I recently rented an Extreme World Formula kart from RPM Racekarts in which I ran a World Formula motor (not that much more power than the 206). I was extremely impressed with the design and the way it handled and I was running the 30mm version. In one weekend I was barely a tick behind the class leader who was racing his kart in which he had a lot more laps and experience with.