The most common and most likely answer to this crossword clue is the 13 letter word DISPROPORTION
I've seen this crossword clue in The Times
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More clues leading to the result DISPROPORTION
Buddy explains!

The straight part
We know that "being the wrong size" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being the wrong size = disproportion.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
DISPROPORTION noun- The state of being out of proportion; an abnormal or improper ratio; an imbalance.
- Lack of suitableness, adequacy, or due proportion to an end or use; unsuitableness.
- To make unsuitable in quantity, form, or fitness; to violate symmetry in; to mismatch.