To resize furniture, you'll first need to add a model to your plan. If you do not know how to do this, see this solution: How can I add on plan something (door, window, model or material)?
You can choose to edit your furniture on the Interior or 3D screens.
Touch the furniture to select it. Once selected, the furniture will appear show a toolbar with buttons Rotate, Modify, Duplicate and Delete. Click Modify button and change the size of your furniture.