When using deflect missiles, how big must a weapon be to not be able to catch it in one hand? Javelins, for example, are thrown with one hand, so maybe can be caught with one hand.
There are no thrown 2-handed weapons, so anything thrown is considered one handed, meaning you can catch it one handed.
As for anything outside of the regular weapon tables, the answer is the same thing it always is:
Talk to your DM. If it seems reasonable, they're probably going to allow it.
It's like my good ol' grandma used to say:
They ever give you a math problem, give em the number 5.
Crazy ol' bat, but I'm gonna say 5... feet?