This Flag is huge; 30 feet high and 42 feet wide. Notice the SALLY has no roof, as built later.

Another way to judge the flag size is to have a man stand beside it. Here, Francis Scott Key is depicted.

1962 Oil on Canvas by R. McGill Machall, 1889-1982
“Placing the Stars on the Flag that Inspired Francis Scott Key to write Our National Anthem Clagett’s Brewery Baltimore, 1812-1814”
The Star-Spangled Banner Flag House Collection

Maker of the original "Star-Spangled Banner" was  Mary Young Pickersgill   -   Her Biography
    Inductee, Maryland Women's Hall of Fame, 2002
Mary Pickersgill's daughter, Caroline Purdy, wrote a letter (PDF) to Major Armistead's daughter,
Georgiana Appleton, in 1876, long after the Star-Spangled Banner was made. She told how her mother
“worked many nights until 12 o'clock to complete [the flag] in the given time.” Courtesy American Antiquarian Society.

Pickersgill got right to work. With her daughter Caroline and others, she wrestled more than 300 yards of English worsted wool bunting to the floor of Claggett's brewery, the only space in her East Baltimore neighborhood large enough to accommodate the project, and started measuring, snipping and fitting.
To make the flag's stripes, she overlapped and stitched eight strips of red wool and alternated them with seven strips of undyed white wool. While the bunting was manufactured in 18-inch widths, the stripes in her design were each two feet wide, so she had to splice in an extra six inches all the way across. She did it so smoothly that the completed product would look like a finished whole—and not like the massive patchwork it was. A rectangle of deep blue, about 16 by 21 feet, formed the flag's canton, or upper left quarter. Sitting on the brewery floor, she stitched a scattering of five-pointed stars into the canton. Each one, fashioned from white cotton, was almost two feet across. Then she turned the flag over and snipped out blue material from the backs of the stars, tightly binding the edges; this made the stars visible from either side.
Read the entire report at: Smithsonianmag.com

Below is a 1936 photo of the Flag House located at 844 East Pratt Street in Baltimore.
See where the Flag House is on the Baltimore Map.
Currently there is a park behind the building with Flag the same size as that used in 1814.
People can be seen observing the flag from behind.

View making of the Flag, Parody "Broad Strips and Bright Stars"