I have a Woodland Mills WC-68 (6” capacity) paired with a L4701. Your MX is a larger framed, heavier tractor but PTO horsepower is similar. (Based on specs I think you may have something like 2 HP more.)
It handles 6” softwood easily (various pines, eastern red cedar, sweet gum, tupelo, etc.). Often have to reduce feed rate for very sappy, gummy stuff such as green pine to keep the discharge chute from plugging with the gummy chips. Variable speed feed is a good thing.
6” hardwood (white oak, hickory, etc.) will slow down after feeding 4’ or so (assuming it’s about 6” for the entire 4’). Pause feed briefly to feed another 4’, repeat until done. IME something that big that doesn’t taper down within a few feet isn’t common so it isn’t a common scenario. 5” and below, will continuous feed with no issue with any species. Personally, I don’t view the brief feed pause at max capacity with hardwood to be a fault. No different than the 12” capacity chipper I used when I worked for a tree service. I do think that indicates it’s pushing the power limits of the tractor so it’s unlikely you’ll get much, if any, more than about 6” capacity in hardwood, possibly a bit more in softwood.
As
@GrumpyFarmer pointed out above, as a practical matter it’s easier to feed a 6” limb into a chipper with an 8” capacity feeder than to feed it into a 6” capacity feeder. May make sense to get a 8” chipper knowing you may not have the HP to use all 8” of that capacity, but will get something easier to feed bigger stuff understanding you may be limited to around 6” with hardwood and more like 7” in softwood. YMMV…
