A “footlong” (or “foot-long,” originally two words) is a twelve-inch-long hot dog. The “foot long” craze began about 1935; by 1938, many restaurants (mostly in the midwest) were serving foot-long hot dogs. Today, footlongs can be found at many state fairs.
 
The Subway restaurant chain began advertising “$5 foot long” sandwiches in 2008. “Foot long sandwiches” (such as the hero sandwich and the submarine sandwich) were called that by the 1940s.
   
The never-seen hot dog documentary Footlong (2002) featured this author. A quite-different “footlong” entered sexual slang terminology by the 1960s-1970s.
   
     
Wikipedia: Minnesota State Fair
The Minnesota State Fair is the state fair of the U.S. state of Minnesota. It has been marketed for generations as “The Great Minnesota Get-Together.” It may be the largest state fair in the United States in terms of average daily attendance, though the State Fair of Texas runs twice as long and is the largest by annual attendance.
(...) 
Food
One of the primary attractions at the State Fair is the wide variety of foods to be had. Many foods at the fair are served “on a stick.” Classic fair foods such as cotton candy are readily available. Many foods reflect Minnesota’s agriculture; cheese curds, milk shakes, corn dogs, and FootLong Hot Dogs are popular favorites. Many foods at the fair are deep fried or come on a stick; from the classic corn dog to alligator-on-a-stick or deep-fried candy bars. New to the fair in 2006 was hotdish on a stick, a variant of a classic staple of Minnesotan cuisine. In 2007 the new food was spaghetti on a stick. 
 
Louie’s Original Footlong Hot Dogs (Tonawanda, NY)
Louie’s Hotdogs is celebrating its 55th year in business! Same owners since 1951.
 
Louis Turco and family welcomes everyone to come visit our Sheridan Park location. The park is getting greener and the footlong hotdogs are on the charcoal grill. Come in and sample some of our award winning “Buffalo Originals”, Curly Q, and Sweet Potato Fries, Veggie Dogs and Burgers. To quench your thirst; Homemade Shakes and Ice Cold Loganberry. Louie’s Hotdogs is “THE ORIGINAL HOME OF THE FOOTLONG HOTDOG”.
 
Wikipedia: Hot dog variations
Port Dover, Ontario, Canada
The Arbor restaurant serves famous footlong hotdogs. Many fresh toppings make this a local favourite, along with the legendary “Glow” beverages.
 
Wikipedia: Subway (restaurant)
Subway is a franchise fast food restaurant that primarily sells sandwiches and salads. It is owned by Doctor’s Associates, Inc. (DAI), and has over 29,045 franchised units in 86 countries as of March 2008 and is the fastest growing franchise in the world. Currently, Subway is the third largest fast food chain globally after Yum! Brands (34,000 locations) and McDonald’s (31,000 locations).
(...)
In 2008, Subway began to offer Five Dollar ($5) Foot-long submarine sandwiches as a limited time only promotion. The fanatical response however, prompted Subway to make A Five Dollar ($5) Foot-long Everyday Value Menu that includes 8 footlong sandwiches; Spicy Italian, Cold Cut Combo, Tuna, BLT, Black Forest Ham, Meatball, Veggie Delite, & Oven Roasted Chicken as a permanent menu item.
 
Urban Dictionary
footlong
A lengthy penis (not necessarily 12 inches, but not under 10), a hot dog, or a sub (See: hoagie). All are good on buns.
Dave: Hey- is that a footlong?
Burt: Sure is.
Dave: You know I’ve got buns for it.
by Lour Apr 24, 2008
 
9 July 1935, Altoona (PA) Mirror, pg. 22, col. 3 ad:
CHET’S PLACE
Royal Pennsylvanians
FLOOR SHOW
Famous for Its 12 Inch Hot Dogs
   
19 September 1936, Elyria (OH) Chronicle Telegram, pg. 6, col. 5 ad:
Foot Long HOT DOGS
The largest and best in town. At the counter or to take out by the bag full, each…10c
DEAN’S LUNCH
 
4 June 1937, Tyrone (PA) Daily Herald, pg. 2, col. 4 ad:
12 INCH HOT DOGS
Dougherty’s Restaurant
   
20 June 1937, Albuquerque (NM) Journal, pg. 10, col. 5 ad:
HOT DOGS
ONE FOOT LONG
FAREWELL INN
 
13 July 1937, Oakland (CA) Tribune, pg. 30, col. 1 classified ad:
FOOT long hot dog stand—Busy corner and only $10 rent.
   
24 September 1937, North Adams (MA) Transcript, pg. 13, col. 4 ad:
AT FRANK’S PLACE
(...)
Hot Dogs one foot long
10c
   
17 March 1938, Tavern Weekly News, pg. 3, col. 4:
12-Inch Hot Dogs
Out at Rockford, Ill., passersby can see a glaring neon sign in the shape of a huge hot dog which proclaims “Hot-Dogs A Foot Long.”  Anyone who cares to verify this advertisement has only to step up to the window and look at the 12-inch hot dogs on display. Try to visualize it if you can! Try to imagine what one of those hot dogs looks like in a bun which reaches both ends of this monstrosity!  And all of the trimmings go with it—plenty of onions, mustard, and pickle.
 
6 May 1938, Waterloo (Iowa) Daily Courier, pg. 11, col. 7 ad:
THE BIGGEST HOT DOG IN IOWA
A FOOT FOR A DIME
MAID-RITE
   
13 May 1938, Logansport (IN) Pharos-Tribune, pg. 12, col. 2 ad:
Foot Long Hot Dogs 10c
AT KEWPIE’S
 
19 July 1938, Wisconsin State Journal (Madison, WI), pg. 10, col. 5:
Davenport, Ia., spots are advertising the “biggest hot dogs in the world”...they’re a foot long, sell for a dime.
   
19 May 1941, Tavern Weekly News, pg. 9, col. 3:
FOOT-LONG HOT
DOGS ATTRACT
Palatine, Ill.—Foot-long hot dogs are featured at the Rainbow Inn, Higgins and Roselle Roads. But, for those who prefer something more substantial, a steak or a chicken dinner is always available.
       
24 March 1944, Marion (OH) Star, pg. 2, col. 7 ad:
Foot Long Sandwich Bar
FORMERLY SHADIX DINER
 
Internet Movie Database
Footlong (2002)
Directors:
Gerry Beyer
Chris Patak
Genre: Documentary  
 
(Trademark)
Word Mark FOOTLONG
Goods and Services IC 030. US 046. G & S: Sandwiches. FIRST USE: 19670801. FIRST USE IN COMMERCE: 19670801
Standard Characters Claimed
Mark Drawing Code (4) STANDARD CHARACTER MARK
Serial Number 77324328
Filing Date November 8, 2007
Current Filing Basis 1A
Original Filing Basis 1A
Owner (APPLICANT) Doctor’s Associates Inc. CORPORATION FLORIDA Suite 306 300 South Pine Island Road Plantation FLORIDA 33324
Attorney of Record Valiere Pochron
Type of Mark TRADEMARK
Register PRINCIPAL
Live/Dead Indicator LIVE