I have used both a 5 and 6 foot rotary mower on my 336d model. They both would mow fine except in very thick, high grass where I could bog the engine at times.
I liked the 5 foot better as it was more the size of my rear wheel spacing, the 6 foot was a bit wider and presented problems if I mowed close to something and I would forget the extra width.
If you mow in tall, heavy weeds watch the radiator, it can quickly clog it up and overheat from the fuzz, seeds etc.