While editing the duration of a clip in the Selected Clip Preview area, clicking the In Point field will open the Enter New In Point dialog, and clicking the Out Point field will open the Enter New Out Point dialog. Once either of these dialogs is open, type a new in or out point into the field and click OK to save the change.

Note that this method of editing clip lengths is best when precise lengths have been predetermined. If you prefer to line your in and out points up to visual cues on the clip, use the clip navigation buttons in the Selected Clip Preview area to cue up the precise moment of the in or out point, then use the red and blue flags to mark the in or out point.