The most common and most likely answer to this crossword clue is the 6 letter word EXTENT
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More clues leading to the result EXTENT
Buddy explains!

The straight part
We know that "Size" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case size = extent.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.
What does EXTENT mean?
EXTENT noun- A range of values or locations.
- The space, area, volume, etc., to which something extends.
- A contiguous area of storage in a file system.
- The valuation of property.
- A writ directing the sheriff to seize the property of a debtor, for the recovery of debts of record due to the Crown.
- Extended.