RE: "this" vs "it"— either can be used, though "it" is more common, and natural. The structure of the sentence could use a bit of correcting, though. Here's how I would write it: They will try to cut all the wood you see in the pile. Do you think they can do it in one day?

WHICH should be changed to THAT, or to be even more natural, don't use either one, as in Elisa's example.

Hi Henry Here what I'd say diffrently: They'll try to cut the wood you see in the pile. Do you think they can do it just in one day? I hope I helped