Click on the unit you want to resize to select it.

The design panel on the left will show all the properties of that cabinet.

Under the "Quick Access Options" tab, look for "Dimensions" section. Enter the required height, width and depth into the H, W and D fields respectively to modify the dimensions.

Or, you can find the same "Dimensions" Section under "Customise" tab as well in the design panel.

For Blind corner unit you will get an additional parameter called. "Shutter Width" which is the width of the shutter added. If you modify this, then the size of the shutter will get changed and as per the same, the blind panel will take the balance dimensions after reducing the shutter width from the total width of the cabinet.

For L-corner units, you will get 4 dimension parameters which are explained in the image below,