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I saw many questions regarding "na" at the end of a sentence; however, what about at the end of a word? Consider these two sentences:
Korewa ōkina kurumadesu. (これはおおきなくるまです) This is a big car.
Ōkii kurumadesu. (おおきいくるまです) (This) is a big car.
What does 'na' (bolded) have to do in this sentence?
Consider this sentence too:
Korewa akai kurumadesu. (これはあかいくるまです) This is a red car.
How come 'na' is used in the first sentence but not in this one. Does 'na' mean 'a' as an article?
In the 連体形{れんたいけい} (attributive form) of a na-adjective, な comes at the end of it, like 静{しず}かな, きれいな.
In the 連体形 (attributive form) of an i-adjective, い comes at the end of it, like 大{おお}きい, 暑{あつ}い. That is the same as the dictionary form.
大きな is the 連体形 (attributive form) of an old na-adjective おおきなり, and only the attributive form is used in the present day. 小{ちい}さな, おかしな are also the same thing. They are analyzed as pre-noun adjectivals but are a bit different from other pre-noun adjectivals.
So both 大きい車{くるま} and 大きな車 are used, and they have almost the same meaning.
「おおきな」を連体詞とする説もあるが、この語は「耳の大きな人」などのように、述語としてのはたらきをもっている点が、一般の連体詞とは異なっている So if they are 連体詞 they are different from other 連体詞 in functioning predicatively, but they are also a little unusual as 形容動詞 since only one form is in use (they are "defective", as Eiríkr Útlendi says).
To expand on Yuuichi's answer, note that both [大きい]{ōkii} and [小さい]{chīsai} are the "basic" -i adjectives for large and small. These are probably the most common forms, and these conjugate fully. See the corresponding inflection tables in the Wiktionary entries at 大きい and 小さい.
Words that are used extremely frequently in any language sometimes wind up with funny forms. Consider "go" in English ‑‑ the past tense is "went", which was originally the past tense for a completely different verb, "wend".
For 大きい and 小さい, these both overlap in modern usage with the older forms [大きな]{ōkina} and [小さな]{chīsana}. These look kinda like -na adjectives, but they can only be used as attributives ‑‑ that means, they must be followed by a noun, they cannot be used at the end of a sentence. Also, they cannot be used as adverbs (changing the [な]{na} on the end for [に]{ni}).
I think these two, [大きい]{ōkii} and [小さい]{chīsai}, are the only two words that follow this exact pattern, with full regular -i adjective forms and a single defective (i.e. missing portions) -na adjective form. As a learner, you should also be aware that there are a few other irregular words in Japanese, such as [同じ]{onaji}.
Happy studying!
Chocolate kindly reminded me of the existence of [おかしな]{okashina}, a similar attributive-only form for regular -i adjective [おかしい]{okashii}. I can't find evidence of any others, but languages are big and time is scarce. :) If anyone knows of more, chime in and let's add them here.
