You can resize a deck opening by clicking and dragging one of its edges.
To stretch a deck opening:
Click on the opening edge you want to move. The entire opening is highlighted, and a blue grab handle appears at the center of the opening edge you selected.
Hover your pointer over the center
grab handle to display the Move cursor.
Click and drag in the direction you want to stretch.
When the opening is the correct size, release your mouse button.