One of the many helpful accessories Windows 8 offers, Microsoft Paint offers basic graphic and photo editing without you having to purchase expensive specialty software. The eraser tool works like a brush, and it replaces color in the image with your chosen background color, which defaults to white. To change the size, click "Size" on the Tools ribbon and choose the right eraser for your needs.
Using the Eraser Tool
To activate the Eraser tool, access the Home tab and the Tools ribbon. Click the icon that looks like an eraser. The Size option appears farther down the ribbon. Hovering your mouse over the image shows you the current eraser size so you know whether to go smaller or larger. Hover again after you pick a different size to ensure it's right for the job before you click to engage the Eraser tool. To change the size using a keyboard shortcut, use Paint's shortcut for changing a brush size after selecting the Eraser tool. Press "CTRL +" to increase the size or "CTRL -" to decrease the size one step.