Translation:There are two big pockets on this shirt.
Consider a situation where you're out shopping with a friend and she's trying to convince you to buy a shirt with no pockets. You've been talking with her about this pocket-less shirt for several minutes. You spot a shirt you like much better, because it has pockets, and tell her:
"This shirt has two big pockets."
That's the point of には. It adds a certain emphasis, highlighting the fact that you're talking about "this shirt in particular," where as に would simply be you making an observation that a shirt has two pockets.
It's both. If I understand correctly, 大きな is used more frequently than 大きい when it comes before the noun (e.g. big shirt: 大きなシャツ, 大きいシャツ) but both are correct. However you would only use the い form when it comes after the noun (e.g. Shirt is big: シャツは大きい, NOT シャツは大きな). And just to clarify, it's never 大きいな in any context.