Set Finger Tool        ⌥8⃣

Use the Finger Tool to change the length of events in the Piano Roll, or move steps in the Step Editor.

Only visible in the Tool Menu for the Piano Roll and Step Editor. Keyboard quick access is ‘T’-‘F’

Resize notes in the Piano Roll Editor in Logic Pro — Apple Support

You can resize notes in the Piano Roll Editor to change their length (duration). Notes can be resized from either their start or end point using the Pointer, Finger, or Pencil tool.