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 :-):