Does Garak appear in any of the numbered DS9 novels(besides A Stitch in Time)? And if so how big is his role?
Off the top of my head, I remember Garak playing a substantial role in Station Rage by Diane Carey. I'm sure he's in others, but I don't recall specifics.
I can only speak to the DS9 books I own in eBook format, but there doesn't seem to be much Garak in most of the numbered novels. At most, he gets a 1-2 page scene, and they're rarely relevant to the plot or give him any real dialogue. He doesn't appear in The Siege, Saratoga, Antimatter, or Objective: Bajor. Wrath of the Prophets and Devil in the Sky briefly mention him or give him a scene that's about half a page in length - nothing substantial, and certainly nothing that does anything with memorable dialogue or character development. He plays in the poker game that's the main plot of The Big Game, but doesn't show up all that much - he has a scene with Lursa and B'Etor, and that's about it. He's referenced a bit in Betrayal, but his only scene seems to be a passing one with Chief O'Brien. He has one very brief but plot-relevant scene in Warchild. So...appear in the numbered novels? Yes. Play any substantial role? Sadly, apparently not much outside of Station Rage (which I don't remember well enough to comment on.)
I'm pretty surprised Garak didn't play a significant role in the DS9 numbered novels. I would've thought he was the one character most - if not all - authors would love to center a story around.
Garak appears in quite a few of the numbered novels, but I don't think in any major roles. The Big Game Betrayal Warchild Proud Helios Valhalla Devil in the Sky Station Rage The Heart of the Warrior Wrath of the Prophets Trial by Error Vengeance The Courageous The Liberated A Stitch in Time