Note, therefore,that you cannot say e.g.
*a too big subject
*a so big subject
nor can you employ the construction at all except with a singular, countable noun (so we may not have. e.g. *too/so depressing news or *too/so busy people.)
In the case of 'so' there is an alternative construction, using 'such (a)', e.g.
It was such a big subject.
which, interestingly, does allow collocation with both non-count and plural nouns, e.g.
It was such sad news.
They were such nice people.
but no such alternative exists in the case of 'too'.