is the default duże if you don't know what "this" is (and therefore don't know the gender)?
Yes , "to" is always neuter. even if you know what it is. Of course if you know what it is you can say: Duża/duże/duży jest.
Basically, you can think of the omitted noun (what's linguistically called the "zero pronoun") as always neuter.
Ok, I thought I had a grasp on these demonstrative pronouns, but now I'm confused. Can 'To' when not being used to modify a noun not be translated as 'that'? Because I answered 'That is big' and was marked wrong.
I saw another comment in that skill that explained 'To' was either this or that. In fact, their explanation compared it to Spanish esto/eso/aquel [this/that/that (over there)] where the Polish equivalent should be to/to/tamto. Is this not correct?
If you want to say for example 'It/This/That is my shirt' you can say 'To jest moja koszula'. There isn't really any difference in translation between 'this' and 'that' in this context so you should report it if 'that is..' is marked wrong. Btw. you can also omit the verb and say 'To moja koszula', but the first one sounds equally natural.
'That' is tamto/tamta/tamten (neut/femm/masc), like the 'this' (or 'it') which is to/ta/ten.
-y is for adjectives describing masculine nouns (duzy kot)
-a is for adjectives describing feminine nouns (duza ryba)
-e is for adjectives describing neuter nouns (duze chleb)
'Chleb' is masculine. A neuter example would be 'duże dziecko' (yes, a child is neuter.. :S )
To - This is Jest - Is or It is duże - big.
Am i missing something or does this not make sense?
Why is jest there if To duze translates to this is big?
"to" is a neuter form of "this" as a determiner. Like in "this wine", "this animal", "this child" (to wino, to zwierzę, to dziecko).
But in such a sentence, "this" is not a determiner, it's a dummy pronoun. And "jest" will be needed in most examples. (Yeah, most. Sometimes it can be omitted, but it's actually hard to pinpoint any rule as to 'when' it can be omitted. But for example you can say "To ciekawe" for "This is interesting" and it sound fine without 'jest', but "To duże" doesn't really sound well).
Można powiedzieć "jakie to duże" (how big it is, in French, "Comme c'est grand")