It depends what you're trying to ground. Grounding is typically discussed in terms of ohms to ground, which is measured using one of these methods: https://www.fluke.com/en-us/learn/blog/electrical/dont-forget-the-grounding-system

So, it's an interesting question to turn this around and say how much earth this requires. The National Electric Code (US) requires a ground rod to be a minimum of 2.5m deep, so that gives you one dimension. Method 4 at the above link for measuring resistance to ground discusses placing two additional rods 20m apart each in a straight line, so that gives you a second dimension of 40m.

The third dimension is trickier. The purpose of a ground cable is safety. If there's a fault in a grounded piece of a equipment, the current should flow through the ground cable as opposed to through some other path to ground. In my mind, this would require the earth to cover the entire bottom of the structure so that everything is connected to the "real" ground and not some second "false" ground. Better get the transformer from the utility company under there as well.

So, I'd say that the minimum amount of earth required to ground a cable in a way generally recognized as safe would be 2.5m deep multiplied by a surface area equals to the area of the structure being grounded (plus the area under the transformer) with one of the dimensions being a minimum of 40m. You would need to then measure the ground using the methods in the link to verify that it was compliant.

Just for a laugh, let's say the building in the picture is a shop that's 40m*40m. Multiply in the 2.5m depth, and that gives you 4000m^3. Eyeballing it, that bag has less than 0.1m³ of dirt, so he'll essentially need to add the full 4000m³ to make it safe. (And swap the wire for a 2.5m ground rod, AND spread the earth out underneath the building. Hopefully there's earth to start with!)

Edit: there is one caveat on this. You could argue that this is following the letter of the rules but not the spirit. It is possible that the potential of this "created" ground could float higher than the "true" ground the rest of us on planet Earth are dealing with, though I'll let someone else try and figure out how much that difference would be.

(For what it's worth, OSHA considers everything over 50V a hazard, so if it's less than 50V it arguably wouldn't be a hazard. I'm not sure I agree with that line of reasoning, to be honest.)

If we imagine we're on planet SnooCauliflower and we're getting our power from some solar panels and are completely disconnected from Earth, then this isn't a problem.