What is the maximum size of missile a monk can deflect? Not catch, just deflect. e.g. A boulder from a trebuchet.
Player's Handbook p.78:
[...] you can [...] deflect or catch the missile when you are hit by a ranged weapon attack.
[...] you can catch the missile if it is small enough for you to hold in one hand [...]
As long as it's a missile from a ranged weapon attack, you can deflect it.
Since you are able to hold (not use) a two-handed weapon in one hand the missile can be pretty big. Unclear cases have to be decided by the DM.
The Trebuchet (DMG, p.256) makes a ranged weapon attack, thus a monk would be able to deflect it. Due to common sense, I would argue that you are not able to hold the boulder in one hand and thus are not able to catch it.