Considering all US untowered airports, what is the furthest distance that E2 airspace (dashed magneta line) projects from the airport? Considering all US untowered airports (50 states), what is the furthest distance that E2 airspace (dashed magneta line) projects from the airport for which it is designated?

Measure from the middle of the airport, or if the airspace is officially defined from a navaid or other feature that lies on the airport, measure from that navaid.  

Example of E2 airspace -- KONP-- http://vfrmap.com/?type=vfrc&amp;lat=44.580&amp;lon=-124.058&amp;zoom=10

On this airport, only the inner circle is E2 airspace.  The outer projections are E4 and don't count for this question-- http://vfrmap.com/?type=vfrc&amp;lat=38.894&amp;lon=-119.995&amp;zoom=10

Beware these 3 airspaces, they are special anomalies, they appear to be all E2 but the projections are actually E4 (see related question Why are KACV and KTLV depicted differently on sectional charts than KSGU, KBIH, and KTPL, even though each is E2 w/ E4 extensions? ).  To my knowledge these are the only 3 of this kind:

KSGU -- St. George Regional Airport, UT -- http://vfrmap.com/?type=vfrc&amp;lat=37.036&amp;lon=-113.510&amp;zoom=10

KBIH -- Bishop Airport, Bishop CA -- http://vfrmap.com/?type=vfrc&amp;lat=37.373&amp;lon=-118.364&amp;zoom=10

KTPL -- Temple, Draughon-Miller Central Texas Regional Airport, TX -- http://vfrmap.com/?type=vfrc&amp;lat=31.152&amp;lon=-97.408&amp;zoom=10

Except for these three anomalies, you can generally assume that a dashed magenta line surrounding the airport whose approaches it protects, with no inner dashed magenta line, denotes E2 airspace.

Don't be shy about posting the longest answer you've found -- if someone else posts longer no big deal.  

Give you best guestimate by measuring on sectional, or check this document for the actual airspace description-- E2 airspaces are listed on pages E-2 through E-141 and have "E2" in the heading, like this: 

"ANM OR E2 Newport, OR
Newport Municipal Airport, OR
 (lat. 44°34'49"N., long. 124°03'28"W.)
Newport VORTAC
 (lat. 44°34'31"N., long. 124°03'38"W.)
Within a 4-mile radius of the Newport Municipal Airport, and within 3.5 miles each side of the Newport VORTAC 357° radial extending from the 4-mile radius to 7.9 miles north of
the VORTAC." 

In any case of doubt, use the document linked above to definitively determine for which airport the E2 airspace in question has been designated.  

State whether measuring in nautical miles or statute miles

The document above uses nautical miles (p. A-2).
Best candidate so far--

Sheridan County Airport-- Sheridan, Wyoming-- KSHR
Page E-92 in the "Airspace Designations" document

The E-2 airspace extends 17.6 (nautical) miles southeast of the airport-- and also 16 miles northwest of the airport  -- last amended 5/31/12

http://vfrmap.com/?type=vfrc&amp;lat=44.769&amp;lon=-106.980&amp;zoom=10