How to translate the famous saying "欲成大器，必先退之"？
The idea here is the same as the set phrase: 以退為進, literally, "To advance by retreating." So a literal translation is "To become a great person, one must first learn to back down / be flexible / compromise / back off / retreat".

Personally, I'd suggest something like "True greatness comes after learning when to let go / compromise" which might be better flowing English.

See also the below:


  懼足下以吾退歸，因謂我不復能自強不息，故因書奉曉，冀足下知吾之退，未始不為進；而眾人之進，未始不為退也。
  
   - 唐·韓愈《答侯繼書》