I have only had two horses with hematomas. One was the size of a football just below the shoulder. The other was the size of a grapefruit at the side of the belly where the girth would be. We did use an icepack on the grapefruit size and it resolved within a week (probably because it became more diffused and was on the belly. The other horse with the larger one did not get any treatment but resolved without leaving a mark. Took several weeks to completely disappear.
We did call a vet but he said other than icing there was nothing he could do.
My then weanling had the same thing happen, but it actually started inside the hind leg and dropped to the stifle area. Xray to make sure no damage and it was finally drained when it appeared it wasn't going to resolve on it's own. Vet made sure there was no stifle involvement and I think we had to redrain 2 months later in a smaller amount and then it was fine. My guy just had a small scar from the scalpel cut. The skin went back like it should....
I had a horse who developed a haematoma around one stifle that looked like there were three grapefruits under the skin. It was huge. I iced it (by standing there and holding the ice pack in place), I gave him bute and I walked him. I quit the ice and bute because the walking seemed to do the most good. My vet refused to drain it.
It took months to disappear completely (it did completely go away). I have a photo of him three months after the incident and the haematoma is still visible, though much smaller (maybe tennis ball sized). He did have other injuries which limited his mobillity (especially in that stifle), and it was winter, so I don't think he was moving around much on his own. I think it would have gone much faster if he had been moving more.