The most common and most likely answer to this crossword clue is the 10 letter word UNMEASURED
I've seen this crossword clue in The Daily Telegraph
U
N
M
E
A
S
U
R
E
D
More clues leading to the result UNMEASURED
Buddy explains!
The straight part
We know that "to be very large" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to be very large = unmeasured.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 UNMEASURED mean?
UNMEASURED adjective- Not having been measured.
- Beyond measure; vast; measureless.