If it's standard font you can use the code {size=100}{/size} or whatever size you so wish.
If it's a webfont or googlefont, which I think it is in your signature, you need to use the style code, like so: {style=font-size:100%}{/style}. Adjust the percentage to whatever size you wish.
Remember to change the vulture brackets {---} for square brackets [---].
Hope this helps