I am sure he might reply but in Way Beyond Monochrome, Ralph Lambrecht discusses what the reasonable diffraction limit is per format; if I recall correctly, for 35mm film it is between f/8 and f/11; for 6x6 it is f/16. Since diffraction is only truly visible in enlargements, larger formats suffer from the same diffraction effect but it is less visible since one does not normally enlarge a negative to the same extent which is what allows large format photographers to use such extreme f-stops (f/128 anyone?). If you enlarged a 35mm to 4x6 and a 4x5 to 20x25, they would exhibit the same diffraction effects. So having a different focal length of lens will not make a difference but since wider lens' tend to give "wider" portraits which objects are not close enough to see the diffraction, they would appear to not suffer as much.

Very generally speaking, a picture with a 200mm lens @ f/22 will show little subject appear matter but make the subject larger thus showing any softness due to diffraction; a 24mm lens @ f/22 will show much subject matter (assuming the same focal length) but smaller and thus hiding softness due to diffraction. If you enlarged the 24mm shot, you would see the same effects of diffraction. It's a little difficult to explain - last postcard round I shot some leaves/flowers at f/32 on a 6x6 format, trying to get sufficient depth of field which printed too soft for my taste. I reshot it at f/16 with camera farther back (to get more depth of field) and enlarged to a greater extent in the darkroom to keep the same print size - much more sharp to my eyes.