How to Adjust a Canvas on MS Paint
When you open an image in MS Paint, it will automatically fill the entire canvas. This saves you from resizing the canvas every time you open an image, but there are times when you need more space. Maybe you want to create a border around the image, or you need room to write a commentary. Resizing the image will resize the canvas as well, but you still won't have any workspace. However, Paint allows you to resize only the canvas without affecting the image.
Open MS Paint.
Point your mouse pointer to the tiny square at the bottom-right corner of the MS Paint canvas. Click and drag this corner to resize the canvas while maintaining the same width-to-height proportion.
Click and drag the tiny square at the right or bottom edge of the canvas to resize only the width or height, respectively.
Click the blue document icon, located immediately to the left of the Home tab and select "Properties." Manually enter a "Width" and "Height" size for your canvas in the fields provided. Click "OK" to adjust your canvas size.