Did you know CafeTran already displays the character count for source and target segments?
Open any project and browse to a segment.
On the status bar (bottom), you see the language pair and two numbers separated with a ": "
It's the source vs the target character count!
Good job for investigating that. A length warning would be helpful indeed in specific projects where the current CT provided info is not enough.
Proof of concept: https://youtu.be/WCqVIto2JKs
Rudimentary Keyboard Maestro macro attached.