For the purposes of this question, consider Alaska’s and Louisiana’s administrative districts to be the equivalent of a ‘county.’ Just looking at Alaska, I found ‘Fairbanks North Star,’ clocking in at 20 characters (including the spaces). I think that will be tough to beat. Any longer?
Happy hunting. 
Index of U.S. counties (Wikipedia)
Includes Municipality of Anchorage, AK (“a consolidated city-borough under Alaska law”)
Aleutians West Census Area, Alaska
Northumberland, in VA and PA.
Alaska “County equivalents” are not really that, because they are fluid and often change their identity and borders. And do not have the governing powers that are afforded to county status.
Not a county but I’ve been to this lake in Massachusetts:
**Lake Chargoggagoggmanchauggagoggchaubunagungamaugg
**
In Virginia, there is “City of Colonial Heights”, and “City of Charlottesville”, which have county-equivalent status in Virginia’s unique Independent Cities configuration;.
If spelled out, there is “Saint John the Baptist” Parish in Louisiana.
I downloaded the list from Wikipedia and used Excel to sort by name length. The results may be a little dirty because they contain superfluous descriptions in many cases but it depends on your definition.
County or equivalent State or district Length
Carson City, Consolidated Municipality of [19] Nevada 45
Prince of Wales – Hyder Census Area[5] Alaska 38
San Francisco, City and County of[10] California 37
Southeast Fairbanks Census Area[5] Alaska 34
Broomfield, City and County of[11] Colorado 34
Nantucket, Town and County of[17] Massachusetts 33
Wrangell, City and Borough of[4] Alaska 32
Honolulu, City and County of[15] Hawaiʻi 32
Fairbanks North Star Borough[4] Alaska 31
Yakutat, City and Borough of[9] Alaska 31
Juneau, City and Borough of[7] Alaska 30
Denver, City and County of[12] Colorado 30
Aleutians West Census Area[5] Alaska 29
Anchorage, Municipality of[6] Alaska 29
Lake and Peninsula Borough[4] Alaska 29
Sitka, City and Borough of[8] Alaska 29
Valdez–Cordova Census Area[5] Alaska 29
Colonial Heights, City of[23] Virginia 29
Hoonah–Angoon Census Area[5] Alaska 28
The question is, in something like “City of Colonial Heights”, do you count the phrase “City of”? Would that be equivalent to counting the word “County” in a more conventional county name? Same issue applies to “Municipality of Anchorage”.
Yeah, for the purposes of the question, I was looking for the name not including the type of administrative district, which most of the ones listed above are showing.
San Francisco is 13 letters, including the space.
Ha! Nice try, Consolidated Municipality of Carson City!
Wisconsin ain’t got nothin’ to compare to the names above.
Maryland has Prince George’s County. Counting the space and the apostrophe, “Prince George’s” is 15.
I filtered out what many would view as bogus or garbage entries to find the longest true (actually used in common language) county or parish name that there is. The winner is:
St. John the Baptist (Parish), Louisiana
I wanted to impress y’all with my ability to count.
Pottawattamie county, ia.
13, thats the largest in ia I believe, across the river from omaha.
I guess Yoknapatawpha County, Mississippi is not quite in the running.
Remember, in most atlases the place names are fully spelled out.
Thus, Saint John the Baptist.
22 letters and spaces. 
Thanks, all. I was working with a file that had a spot for county [or other administrative district type] name with a maximum of 22 characters, and was wondering if there were any that might have exceeded it.
Pottawattamie County, Iowa and Chattahoochee County, Georgia both have thirteen letters. But as jtur88 noted, the winner for longest single word is Northumberland County (or Counties; there’s one in Pennsylvania and one in Virginia) at fourteen letters.