I dunno if you ever found some horse-sized wolves, but I finally got some of the Thunderwolves myself. I think I was pretty much right, if they had boring static poses, they would fit on cavalry bases, but since they're all dynamically posed either lunging or turning, they overhang.
If you look at the proportions of a Thunderwolf, they're similar size to a Bretonnian horse, but with much larger heads and broader shoulders, but the overall body size and height is similar. They certainly look a lot heavier than horses.
I'm actually thinking of using a Thunderwolf in my Orcs and Goblins army, putting in a chariot base with a goblin hero standing on it's shoulders holding reins and a couple of goblins sitting in the saddle or hanging off the side of the wolf and calling it a goblin hero mounted on a chariot. Compared to the old wolves used for wolf riders, a thunderwolf looks like it'd be totally capable of dealing twice the damage + impact hits and absorbing several wounds in the process, so I think it'll fit nicely as an "act as" chariot.