How to increase line thickness or outline in Windows 8 Paint?
Increase the border thickness of an image using the size option.
Option 1: In Home tab, select Size→ Choose the required border size.
Option 2: Use the shortcut key to increase the size press Ctrl++ and to decrease the size press Ctrl+-
In the above given image, the rectangular shape is selected and the size of the border is adjusted to 5px(pixels).