"a big bank"
Either one is perfectly fine. Both should be accepted for this question.
Supposedly there is a slight difference in nuance, in that 大きな can have a subjective feel while 大きい is more objective. But both have the same meaning and both are grammatically correct.
I'm not a native nor would even call myself a speaker of Japanese. I can't feel the difference in nuance myself. This is just what I've learned from reading about it before.