The most common and most likely answer to this crossword clue is the 9 plus 2 plus 1 plus 11 letter word KNEE-HIGH TO A GRASSHOPPER
I've seen this crossword clue in The Times
K
N
E
E
-
H
I
G
H
T
O
A
G
R
A
S
S
H
O
P
P
E
R
Buddy explains!
The straight part
We know that "really small" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case really small = knee-high to a grasshopper.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.