Click on the Room Divider that you want to change the length of.

In the properties panel on the left, you will find a field called "Change Length."

Depending on the direction in which you want to change the length (A or B), click on the "Change at" dropdown menu and select the desired option. The same will be denoted on the sides of the room divider.

Enter the new length you want for the Room Divider.

Press Enter to apply the changes. The Room Divider will adjust to the new length.