We never got any clear indication in Voyager how big the Hirogen really are.
But, they did appear both in season 4 and season 7, at points in space 30000 light years away from each other. And it's implied they controlled most of that giant communication network that Voyager blew up in season 4. They are a more powerful race than is ever stated directly in Voyager.
Against the full force of the Dominion fleet the Dominion probably wins, but assuming they wouldn't be able to send their entire fleet to the Hirogen front, I think the Hirogen would win. Also the great link would be a lot more vulnerable to them than the Cardassians and Romulans.