There are several grammatical points about this construction. Firstly, note that the adjective precedes the article here. The adjective is modifying a whole noun phrase, not a nominal ( a nominal is just the smaller phrase within a noun phrase that occurs after the determiners or articles). So we see:
Secondly you will have noticed that this adjective is itself modified by an adverb. Now this adverb must (a) be an adverb of degree and (b) must be a deictic word, in other words it is understood by reference to the immediate environment of the speaker, or through some other element of the discourse itself. Simply using a normal degree adverb will not work here:
- *He was very good a footballer as ...
- *He was extremely good a footballer that ...
The three adverbs that can be used here are so, too and as. Some grammarians have also included more, less and enough in this list, but the grammar of these adverbs is in fact significantly different.
The adverbs so, too and as are degree adverbs that cannot themselves give us any idea of actual degree or extent involved. We could think of them as a kind of 'pro - degree adverb'. These adverbs require some kind of benchmark for us to appreciate the actual degree involved. If this information is not provided by the context this normally entails there being a Complement phrase which indicates the actual extent or degree involved. We can consider such sentences in this way:
- It was (X)big a problem [that we gave up the whole project]
Here (X) represents the degree involved. On its own (X) does not tell us the actual degree of the bigness of the problem. It is the clause in brackets which explains the actual extent of the size of the problem.
The adverb involved will dictate what kind of phrase or clause can function as the Complement. The adverb so can take preposition phrases headed by the preposition as or finite clauses typically using the subordinator that. In the sentence above the only possible adverb we could use instead of (X) is the adverb so.
The adverb as typically takes phrases headed by as. It cannot take clauses headed by that.
The adverb too takes to-infinitival clauses, headed by for if they include a Subject:
- He was so big an idiot [that he wasn't allowed to speak in public without his advisors].
- It was so forceful a blow [as to fell his opponent].
- He is as great an actor [as has ever graced this stage].
- He is as good a footballer [as the next]'
- He was too valuable an asset [to let go].
- It was too dangerous a project [for us to take it on].
The preposition as (as opposed to the degree adverb) introduces equality with what follows it. More precisely it indicates some kind of benchmark which is met or exceeded.
Notice that we cannot generally put adjectives before articles, the following are badly formed:
- *big a footballer
- *clever an idea
This only occurs when the adjective is being modified by an adverb. Why? I don't know. I've been looking into this for quite a long time but haven't been able to find out.