To resize a product, you must choose the individual product to resize. Click once on the product to select the entire gallery; click a second time to select the single product. You can now click "r", or click on the Resize Product button on the bottom left.
When resizing a product, you'll see a screen that looks like this:
You'll see the current size selected. All you need to do is click "Select" on the size you want. You'll see the size on the room photo change like this, and just click "Apply Resize" when you're happy with the size. You can click and drag the resized piece around on the wall if you need to before clicking "Apply Resize". You can choose to use the Return/Enter key on your keyboard if you'd prefer!
Please be aware that the only sizes you can use in the Designer are sizes of products you have built in Manage Products. If you don't see the particular dimensions you want, that means you need to build that size of that product in Manage Products to show it in the Designer (but make sure you can actually produce that size first!!).