First, check on your spelling. This is important. The words you're trying to use is spelled 大きい and 小さい*. Both of these are い-adjectives.
Second, it's really easy. As I said above, check the spelling. That already tells you what it is right away.
Now, the breakdown on spellings: い-adjectives are just that. It's virtually every adjective that ends with the okurigana, い, except 2 - きれい and 嫌い (because everyone hates exceptions). These are actually な-adjectives. That's it!
な-adjectives are basically everything else. Even words that are only in kanji!
There's also a third classification, the irregular adjective, which is basically only 1: いい. That sounds confusing, right? It sounds like I should have mentioned this earlier, but believe me, it's not.
The reason いい isn't an い-adjective, and neither a な-adjective, is because it's the only word that sets it apart from the two. When it's conjugated, instead of messing with the last vowel, the whole word changes. It's actually a modern version of 良い (よい). So everything is based on the form of that word, rather than いい. Don't ask why. Memorize.
*EDIT: I had assumed you meant 大きい and 小さい. The post below is also correct, referring to their な-adjective counterparts, which are good examples to also practice. Some words have these versions of adjectives, and the same categorization applies as I explained.
The tricky thing particularly with な-adjectives as opposed to い-adjectives is that you must attach the な when you're describing a noun. い-adjectives do not use な.
