The riddle
What Word Becomes Shorter When You Add Two Letters to It?
From The World's Best Conundrums and Riddles of All Ages, 1924
Short plus er
Add the two letters e and r to short and the result is shorter. The word has grown from five letters to seven, yet it now means "more short". That is the whole trick: the riddle asks about length in meaning, and a listener counts it in letters. The ending -er is how English forms the comparative of many adjectives, as Merriam-Webster notes, and a comparative of short can only mean shorter than before.
How the old books worded it
Today's question says two letters, but the riddle is older and the books say syllable. Riddles and Jokes, printed in London in 1863, asks, "What word is that, to which if you add a syllable, it will be shorter?" and prints the answer as "Short (short-er)". Charles B. Eaton's Riddles and Their Answers (1890) and J. Gilchrist Lawson's 1924 collection ask nearly the same. The ending er is one syllable and two letters, so both wordings describe the same word.
A cousin in a 1903 book
Dean Rivers's 1903 book Conundrums, Riddles and Puzzles asks for words that can be pronounced quicker and shorter by adding another syllable, and the answer is "Quick and short". Quick plus er gives quicker, which is quicker in meaning as well. The pair shows the same trick used on two adjectives.
Words that fail the test
Not every adjective works. Add er to long and the result, longer, means more long, not less. Tall becomes taller. Only words whose meaning is about being small or brief pull the length down. A group can sort a list of adjectives into those that shrink and those that grow, and short is the one that gives the riddle its point.