International 1066 with Year Round Cab, Kioti DK 45S with Cab, 451 Loader
Are you talking about a transport disc or a three point hitch? I am pulling a 10 ft. transport disc with my 45 and it pulls it well (In 4wd.). Just my opinion but I think you would be safe with an 8 ft. but be sure you can lift it easily if it's 3 ph. Not sure what your 3ph capacity is with the 40. If you have a loader I would take the bucket off and leave the loader arms on for extra weight up front.
My NHTC40DA will pull our 6-1/2ft 3point disc cutting almost to the hubs - needs 4wd low & about 2500 rpm to do so (this is in already plowed ground). Our soils are pretty heavy but not gumbo.. Your soil conditions & moisture will affect how yours does.
International 1066 with Year Round Cab, Kioti DK 45S with Cab, 451 Loader
Just feel that the more weight you have up front when pulling heavy ground engaging implements the better off you are. I will be putting 600 lbs of weights on the front of my old IH to pull a 4 bt plow and a disc.
i don't think you're going to have any trouble with either a 7/8' harrow. like TwinWillows said, soil condition will determine what you can pull. i've attached a pic of my DK55 pulling a transport harrow (think it's 10'.. never measured it).