If you want to set a custom text size because the size is too small or too big, read this
In Windows 8, the text size can be changed as a percentage of the normal size. The custom text size setting will provide a scale in form of a slider so that it is easy to change the Text size. Changing the text size is useful when using a projector or when you connect your Windows 8 devices with a TV
1. Step Open Control Panel-> Click Appearance and Personalization.
2. Step Click Display. Click Set custom Text Size.
3. Step Click Set custom Text Size.
4. Step Now Custom Text Size Setting window will appear. Scale to this percentage of Normal Size defines the percentage of the size . Click on the scale and drag left or right. You’ll see the percentage of size inside the box changes. Set the size you want. Click OK