Question: Okay, I just asked a [question](https://worldbuilding.stackexchange.com/questions/82436/would-humankind-remain-the-dominat-species-in-a-world-with-twice-as-big-animals), in which mother nature transformed all animals (or brain containing living beings, if that fits better for you) into a being twice as big in order for them to kill of humankind, recovering the ecosystem.

Well, with twice the heigth, length and width, it's actually eight times the size ($2^3$).

The question was, if humankind could remain as the dominant species and the answers made it clear: Yes, easily.

But it got me thinking. Would the result be the same, if all animals would have turned twice as big at the year 0?

The rules are the same: 

* This transformation happened worldwide at the same time and only took about a minute.
* The physical abilities (strength, speed etc.) scale accordingly to the size and the animals are able to handle the gained mass just as good as their reallife counterparts.
* Mother Nature DO provide the earth with enough food to feed the bigger animals. After all, she wants to keep her children alive, killing only humankind. 
Well, carnivore don't need changes, as their prey scale accordingly. Let's just say the vegetable food grows faster, there is more of it or it provides more of whatever keeps the animals alive. (Doesn't really matter, the question is about the fight between humankind and animals)
* The earth was EXACTLY ours until the year 0.

With all that: Would humankind still be able to become the dominant species in the year 2000 (or earlier), or wouldn't they be able to handle the changes?

Answer: 
I'm going to be a contrarian on this one. 

Hypothetically, if the point is to wipe out humans, would we even be capable of using animals for agricultural work? Without machinery, farming for large populations becomes difficult to sustain. While we could always hunt for meat, plowing is EXTREMELY energetically intensive, and there wouldn't be enough "wild food" to sustain human populations at 1 A.D. 

Early hominids were grouped in small populations and fairly mobile in order to follow their food sources (excepting food rich environments like inhabited jungles). Additionally, EVERYONE was taught the skills to survive, and specialization was comparatively mild.

The sudden cessation of all agriculture would lead to mass die offs. Even assuming slave labor, farmers would have a hard time plowing and planting crops, since humans are weak in comparison to domesticated mammals. Basically all city dwellers would die, since they lack basic survival skills. Soldiers and the like might learn to hunt large game, but military tactics and formations used against large groups of humans don't translate well to individual animal quarries. Regardless, a population sated with meat would still suffer nutritional deficiencies. Your empires would fall without their bread baskets, and while small populations of subsistence farmers might remain, it's likely that genetic inbreeding would eventually lead to gradual extinction.

***TL;DR:*** Early humans survived against megafauna due to small populations, high mobility, and lack of specialisation. By 1 A.D., our accumulation of technology means none of these factors hold. If we lose the capacity to engage in agriculture due to the larger farm animals "un-domesticating," then it is likely that humans will slowly go extinct.
Pretty sure this will be closed soon. So I'll try to help you. First, you really can't just double the sized of animals and have them move or even exist the same way. As you pointed out their volume goes up with the cube while their surface area only goes up with the square. This is a big deal for energy exchange. Also, why didn't the humans double in size? Anyway, none of that really matters. Humans rule because we can manipulate tools. We routinely bring down larger animals (and sometimes fall prey to them) with all of them the size they are now. We still win. I use the term 'win' loosely here, since I can't say for certain that our reign will be as long or as fruitful as those that preceded us.

Also, if you are going to write this into a world, you should really work on your grammar. I'm not intending to be cruel here, just pointing it out. Sorry.