What effect is created by this combination of concrete and mystical language at the beginning of The Scarlet Letter?In describing the scarlet...

What a wonderful observation you have made in the confines
of Nathaniel Hawthorne's "The Custom-House" section of The Scarlet
Letter!  Let's look at the instances of both concrete and mystical language
you mention.  We'll take the concrete first:

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It was the capital letter A.  By an accurate
measurement, each limb proved to be precisely three inches and a quarter in length.  It
had been intended, there could be no doubt, as an ornamental article of
dress.

Concrete details,
indeed!  An exact measurement and description of its absolute use!  It is significant to
note here that Nathaniel Hawthorne actually worked in the Boston Custom House in the
mid-1800s.  It is also important to realize that Hawthorne had Puritan ancestors and
many critics believe that he felt guilt associated with their cruel tactics of penance
and punishment.

Here is the mystical language (where the
word "mystic" is actually used) which immediately follows the concrete
above:

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Certainly there was some deep meaning in it most
worthy of interpretation, and which, as it were, streamed forth from the mystic symbol,
subtly communicating itself to my sensibilities, but evading the analysis of my mind.
... I happened to place it on my breast. ... I experienced a sensation not altogether
physical, yet almost so, as of burning heat, and as if the letter were not of red cloth,
but red-hot iron.  I shuddered, and involuntarily let it fall upon the
floor.

So, the speaker
(Hawthorne himself?) in "The Custom-House" stands there contemplating the letter and,
when involuntarily placing it in the exact play where Hester wore hers, the speaker
feels a burning sensation!  We have entered into the realm of feeling and sensibility of
the author.

Now to approach the point of your question.  It
seems to me if you combine the two sets of language (the concrete and the mystical) the
basis serves as a sort of historical context to the novel as well as a precursor to the
symbolism of the letter.  In short, "The Custom-House" itself, and particularly these
excerpts I've mentioned, give the story verisimilitude.  It makes us believe the story
of Hester.  It gives the speaker, dare I say Hawthorne himself, a connection to the
story and he, being already moved by the burning sensation of the letter, sets up a nice
bit of suspense for the reader who must surely ask, "Why, oh why, does this
happen?"

It's the truth of the concrete details coupled
with the mystery of the burn that provide suspense.  Even further than the wondering
why, the reader should enter into the advanced thought that the letter must surely
"mean" something.  It must be a symbol in this novel, as it also graces the novel's
title.  And it does, dear reader, at first it symbolizes "Adultery" and later it serves
to symbolize Hester being "Able."  God bless her!