The most common and most likely answer to this crossword clue is the 12 letter word COMMENSURATE
C
O
M
M
E
N
S
U
R
A
T
E
Buddy explains!

The straight part
We know that "It’s well-proportioned" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s well-proportioned = commensurate.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 COMMENSURATE mean?
COMMENSURATE verb- To reduce to a common measure.
- To proportionate; to adjust.
- Of a proportionate or similar measurable standard.
- Describing a crystal in which every atom or molecule is placed in the same relative position