I don't know if Y-20 had cabin dimensions are published anywhere, but this image of mountain cat atv being loaded is a fairly good starting point to estimate cabin width. The ATV model at hand seems to match the images of models that are 1550 mm in width.
If that is true, then the width of the floor seems to be 3.875 m.
Width of the floor plus width of the stepping surfaces on which the seats are placed seems to be 4.29 m.
Wall width might be a bit more than that, though it's hard to say as I'm sure wall width is not uniform along the entire length of the cabin. But that's not even important as the width of the ramp entrance (not the ramp itself but the entrance walls) into the plane seems to be somewhere in between the 3.875 and 4.29 m measures.