Translation:A big bank
Here's a useful link that explains why 大きな is a thing even though 大きい is an i-adjective: https://japanese.stackexchange.com/questions/1853/i-adjectives-used-as-na-adjectives-is-there-a-difference-e-g-%E5%A4%A7%E3%81%8D%E3%81%84-versus-%E5%A4%A7%E3%81%8D%E3%81%AA
From what i can tell from the link, ookii and chiisai can be turned into the na form but only as an attributive adjective. So 大きな銀行 is okay but you can't say 銀行は大きです。In that case you use the regular い form, 銀行は大きい。大きい銀行 is also perfectly fine. 大きい is an i adjective and can always be used as such.
Here is another stackexchange question that may be more helpful on that issue: What's the difference between 大きいケーキ and 大きなケーキ?
To summarize the basic findings: Both ōkii NOUN and ōkina NOUN are grammatical. The latter is much more common. There isn't any very obvious semantic difference, but ōkina NOUN may suggest more of a subjective judgement.
There are two types of adjectives: -i and -na. -i adjectives don't need to be modified to describe a noune, for example: kore ha ookii tatemono desu. (This is a big building) On the othe hand, -na adjectives require the -na suffix to describe a noun, as in: shizuka na kuni desu (It is a calm country).
However, if you want to use the verb "to be" (desu) to give characteristics of the noun, -na adjectives won't require the -na suffix and you only change the verb's form depending on the tense (present/past and positive/negative), whereas -i adjectives need to be conjugated depending on the tense.