Tarkus swings his blade down and slices off the side of a cliff, pretty simple, this has been done numerous times before.
Dimensions
Scaling off the tallest figure there, Jonathan Joestar, who has a canon height of 195cm.
The length from side to side is about 2383.33333333333332cm.
The height from the center top to bottom is 3510cm (Of course the height isn't completely uniform, the left side is lower, while the right is higher with a clear round elevation increase, the center is between both though, so this serves as the average overall).
To clarify, the reasoning for the depth and width here is that we can see exactly where it was sliced, with the place cut having a clear, clean texture opposed to the rest of the cliff face where it wasn't cut, telling us where exactly the slice extended down to and how far. Plus the giant energy-esque split that emits light being a thing tells us as much already that there's something going on here beyond the norm.
For our width, as a low end and a high end, we'll scale the width off of Will and off of Tarkus' blade.
From Will, using the average male height of 175cm (No canon height, just seems to be of an athletic stature and around the height of the other male characters bar Jonathan), gets the width to be 4.0697674418604651cm.
Using the length of Tarkus' blade, which is stated to dwarf Jonathan's height of 195cm, gets the width to be 5.1315789473684211cm.
Volume+Yield
Plug these into the volume calculator, to get 34045639.534884 cm3.
Or
Treating the material destroyed as violent fragmentation, 69jcc.
34045639.534884cm3 x 69jcc = 2349149127.906996 joules, 0.56 tons of tnt.
or
42928223.684211cm3 x 69jcc = 2962047434.210559 joules, 0.71 tons of Tnt.
8-C, less than what it is listed as currently but still 8-C.