I'll repeat what I said in the comments.
Firstly, the meaning of "She's 6 feet tall if she's an inch" is not "She is 6 feet tall, which is very tall", but "She's definitely 6 feet tall" or "I'm very sure she's 6 feet tall". That is, it's an emphatic version of "She's [at least] 6 feet tall", with the emphasis being on the truth of the statement, not necessarily the great height. As such, in usage it typically (but not always) follows a statement made by someone else (borrowing from mgkrebbs):
"She looks 5' 10'', don't you think?
Five ten?! She's 6 feet tall if she's an inch!
The purpose here is not just entertainment, but rhetorical effect: it's saying "I'm as sure of her being 6 feet tall as I'm sure of her being at least an inch tall" — and since I am obviously very sure she's at least an inch tall, I'm also very sure she's 6 feet tall. It's equating her being 6 feet tall to her being an inch tall: saying (not necessarily with justification, but that's rhetoric for you) that if you deny that she's 6 feet tall, you must also deny she's at least an inch tall, which you obviously cannot do.
The use of "logic" in rhetoric
More generally, the use of such implication sentences in rhetoric seem (to me) to be of two kinds. If you want emphasise a statement X (say emphatically that X is true), you can pick either the form
- "If T, then X" (= "X if T")
or
- If not X, then F (= "X or F")
where T is some trivially true statement, and F is some trivially false statement. Examples of the "X if T" type include the ones you gave:
She's 6 feet tall if she's an inch.
It was a gallon of blood if it was a drop.
The baby was 10 pounds if it was an ounce.
She's forty if she's a day.
If I've told you once, I've told you a hundred times.
If I'm not hallucinating, I heard you come in at 3 am last night.
Examples of "If not X, then F" type include:
If he's a doctor, then I'm the Pope.1 (X = "he's not a doctor", F="I'm the Pope")
The prices will fall next week, or my name isn't [Name].
If he can hit a ball, then pigs can fly.
If that's pure gold, then the moon is made of cheese.
Not very good examples I'm afraid, but you get the idea.
[1: Entirely unrelated aside: There's an anecdote that once, when Bertrand Russell mentioned in a lecture that starting from a false premise you can prove anything, some smart alec said "1=2, prove you're the Pope". Russell immediately replied with "The Pope and I are two, therefore the Pope and I are one".]
This rhetorical device can of course be entertaining or humorous, as in Dorothy Parker's poem Comment:
Oh, life is a glorious cycle of song,
A medley of extemporanea!
And love is a thing that can never go wrong
And I am Marie of Roumania.
The earliest use of it in print that I can find is from The New Monthly Magazine and Literary Journal, 1833. It is from a "paper" titled "The Story of Hester Malpas," by L.E.L.:
Hester now looked at her aunt, who was the very reverse of what she had imagined : she had always thought she would be like her father, and fancied a tall, dark, and handsome face.
According to an excellent post found at Wordwizard.com, and as seen in the citation above, the phrase was originally used to describe women as well as men up until the early 20th century (see EL&U discussion here). Also of note, the popularity of the phrase spiked after a movie by the same name was made in 1941 starring Caesar Romero.
Best Answer
Here's a possible early example from The American Museum for February, 1788:
A 1752 printing of William Shakespeare's Julius Caesar includes tall-tale:
Modern copies render this as:
So the 1752 was probably a typo, but could the mistake have been influenced by the existence of tall-tale?
The same applies to this 1714 printing of The Life and Death of Richard III: