"It is much bigger."
Translation:Tá sé i bhfad níos mó.
You would only use tá sé - "bigger" is an adjective, and the basic use of the copula is is for linking two nouns, not a noun and an adjective.
Sigh, just when i thought I was getting the hang of word order. I had put "i bhfad" at the end, assuming it is like an adjective that goes after the thing it describes....
How would I normally decide if "nois mó" means "bigger" or "more"? Is it just a matter of context?
"more" is usually a determiner or an adverb, while "bigger" is an adjective. Adjectives typically follow the noun in Irish. So Tá níos nó madraí ná cait ann means "there are more dogs than cats", but Tá madraí níos mó ná cait means "dogs are bigger than cats".
In many cases "more" and "bigger" are basically interchangeable in English - "we need more staff"/"we need a bigger staff", "I want a bigger breakfast"/"I want more breakfast" but the adverb/adjective distinction still applies.