Appropriate dash to use when attributing a quotation?

If I’m citing a poem or quotation, what kind of dash precedes the author’s name?
For example:

This Business of Printing; which I am heartily tired of, and repent I e’er attempted.... 

—John Baskerville

Should this be an em dash, an en dash, or something else?
And should there be a space between the dash and the author’s name?

Given these choices:

U+2010 ‭ ‐  HYPHEN
U+2011 ‭ ‑  NON-BREAKING HYPHEN
U+2012 ‭ ‒  FIGURE DASH
U+2013 ‭ –  EN DASH
U+2014 ‭ —  EM DASH
U+2015 ‭ ―  HORIZONTAL BAR
U+2212 ‭ −  MINUS SIGN
U+2E17 ‭ ⸗  DOUBLE OBLIQUE HYPHEN

The right answer is actually U+2015, whose alternate name is indeed “quotation dash”. Failing that, you are supposed to use U+2014. This is very common in Romance languages, BTW, using a quotation dash for speech quotes.

Note that even Bringhurst, who isn’t a fan of the long em dash, rightly says to use two of them for bibliographical entries. The recently released Unicode 6.1 has given us two more dashes to help with this:

U+2E3A ‭ ⸺  TWO-EM DASH
U+2E3B ‭ ⸻  THREE-EM DASH

I assume the style to which you are adhering, if any, does not prescribe a specific type of dash, and therefore this is a matter of personal preference and aesthetics. I would definitely not use an en-dash; I reserve en-dashes strictly for (usually numerical) ranges. Therefore, I would prefer to use an em-dash. I personally never put spaces around em-dashes—e.g., when using them to enclose a parenthetical phrase—so I would suggest not using a space.

You might get some more answers by cross-posting this question to https://tex.stackexchange.com/ (a StackExchange site dedicated to typesetting).

Edit: I am dismayed to report that it appears as if StackExchange uses an en-dash in comment signatures. This is a grievous error! We should all file bug reports! ;-)

I have been searching for the same as the OP. More searching has revealed this in the U.S. Government Printing Office Style Manual (I have not read the whole of it, so I might be misinterpreting it):

16.17. Signatures, preceded by an em dash, are sometimes run in with last line of text.

UPDATE (2019-10-01)

There appears to be a PDF render of the manual with visible examples: https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2016/pdf/GPO-STYLEMANUAL-2016.pdf