@ajkrishock Like reconnnn said, there's no way to change text label size, but you could use an image as one workaround. Basically, make an image file with the text you want and a transparent background, then use scale="fit" on your button code. This would take the place of the text label:
<button image="text.png" scale="fit" />
The built-in icon sizes can't be increased/decreased, but if you download the original icons themselves from Font Awesome (linked above), you can then save them as whatever sized image files you want and use them as button images.
For both of the above solutions, just increase/decrease the padding in your image to modify its scaled size as a button image.