Okay, so in my story I plan on having centaurs that evolved from hyracotherium (otherwise known as the “dawn horse”). They started off with a mutation that gave them a second pair of limbs which while not useful at first but over time would later evolve into their famous “human torso” allowing them to reach foods higher off the ground such as fruits and nuts. For all intense and purposes their lower halves are similar to that of a horse except with a slightly arched back so as to not pinch the spine and nervous system between torsos and 4 hoofed toes on each foot. Now with that said, how big could my centaurs be to be possible to live do to the square cube law? Would the entire body have to be smallish? Could I have a human sized torso alongside a horse sized lower body? Would the “human half” have to be shrunken or the horse half? I’m so confused as to which I should do so any help would be greatly appreciated.
If you don't mind changing the proportions of the underlying animal, your "centaur" could be a bit bigger than a modern African elephant.
The big thing square cube law limits is how slender the legs can be for a given animal size, and how big the chest cavity needs to be to house large enough lungs to oxygenate the body (and brain). If you allow legs as thick as those of a Columbian mammoth (from the past Ice Age, slightly larger than the better known wooly mammoth, and extinct not long after humans entered the New World, though still in question if there's a causal link there), your centaur might weigh as much as twenty tonnes, stand more than four meters at the shoulder, and of course project the head and arms above that, perhaps ending with a total height of six meters or so to the head.
Of course, such an animal won't move like a horse; it'll move like a mammoth (which we presume moved a lot like an elephant). If you insist that it move like a horse, you're looking for a size similar to a destrier or heavy draft horse -- perhaps 18 hands (six feet) at the shoulder and another couple feet to the head, as per @JonSG answer.
Head weight in general is estimated to be in the range of 10% of the weight of an animal (according to the interwebs anyways: https://neeness.com/how-much-does-a-horse-head-weigh/)
We have drafts that are 2000 pounds and by the prementioned wisdom might support traditional heads in the 200 pound range. I can assure you that a casual nod can send one flying if you are not paying attention.
If the head weight was allocated to a human torso and head there would be plenty of capacity to spare. I'm estimating that a rather big human torso could be placed on the body of a draft.
Of course, there might be caloric issues to deal with as a centaur would have to be more or less constantly stuffing it's human face with food.
For additional context, check out this Percheron and rider. The rider from the waist up is clearly a fraction of the mass of the horse from the withers up.
I estimate a Dwayne "the Rock" Johnson torso and head on that Percheron would be no problem and would stand 7 to 8 feet at the human head.
Here is my guess of what that looks like :-):
If you're changing the proportions of a centaur, then I will open with a major mechanical issue that's basically known only to relatively experienced horse riders.
Centaurs have a permanent curse by horse standards, they are, by design, permanently very heavy on the forehand.
This means that they're going to be off balance, slow with short strides, have pain in the front legs and hooves, and have a tendency to trip and stumble. The hollow back you describe is also a symptom of this.
Normally you can train a horse out off the forehand, but the fundamental extra weight, no matter how much you attempt to reduce it by shrinking the human part of the torso, will cause it to remain an eternal problem.
So the direct answer to your question is shrink the human part as much as possible, make it nothing more than a glorified long neck with some muscle structure to support the head and probably weedy arms. The practical answer is to handwave. Once you start picking holes in the standard centaur people realise there are more and bigger holes to account for, if you stick with the standard model of centaur then it's just accepted as a known mythical creature.
As with many such things, the secret is not to look too closely, or you might see the stitches where the mermaid has been sewn together in the middle.