Through Karsa Orlong's storyline we see him riding his Jhag horse Havok (At first his fathers one and then later the one he gets on the Jhag Odhan). I estimate Karsa to be eight to nine feet tall and so I was wondering how big Havok, and Jhag horses in general, would be.
According to the Malazan Wikia:
Jhag horses were originally bred long ago by the Jaghut, they were carnivorous. They were usually grey as iron with black manes and tails. Twice as large as a regular horse they were larger even than the horses the Teblor had bred. The mares were broad-backed. They were drawn to the smell of bloodwood and blood oil.
So, basically twice the size of a normal horse, meaning really big.