My experiences are with the Med and Small. Being near the bottom of the suggested weight range in the M, the second from the stern position worked best and performed just as well if I added 25+ lbs of gear in back.
Then I switched to the Small, which I am at the top end of the weight range for. Initially, I went to the second to rear seat position as I had used in the M. I discovered that was not appropriate for my weight and build. All the way forward with the seat is required instead. Plus, I can only load minimal weight in the stern without noticing a loss in performance. I've got it outfitted were I am perfectly comfortable in the small and the performance is phenomenal when not loaded with much gear, but it required removing the seat pad and thinning the knee pads to allow more room in the cockpit. That gained me an inch in increased deck height and lowered my center of gravity.
I suspect that you could do the same for yourself in the M and may find the seat all the way forward is best, being you too will be at the top of the weight range for that size. However, for your size I'd recommend you go with the Large. Trust me, all of the sizes of Karma each drive like they are smaller. The perform big in stability, but are agile like being smaller. I have numerous friends your size that are in heaven in the Large. Their boating ability is better than ever and they are all smiles.
Just know that the primary and secondary stability of the Karma are far better than other boats. The boat will get you through the gnar far better than you've ever experienced with other designs and will dart in and out of micro eddies like a Porsche. Plus, all of the Karmas roll up super easy.
No risk, no reward. It is not that we have to, it is that we get to. Preparation and education are essential to self-confidence and success. - KV
"If there is no risk there is no adventure."- Bill Briggs