"This is big."
Translation:To jest duże.
"To" in such a context like in this sentence is always described by neuter adjectives, similarly to "ono" (it).
If you had a whole noun phrase (duże jabłko, for example), it would be ok. But with an adjective, you should use "jest".
There would be some contexts where "To duże" would make sense as a short answer to something, but they rather wouldn't be "This is big" in English. And they are rather limited.