enlarge text ?

How can I enlarge text when typing on facebook. nothing on my Samsung Galxy T580 and nothing I can find on face book on which text when typing is around 7 very small. I can't understand why it does not have same size level as this text on here. Yet when posted on facebook the text is lager and readable.
on here you can do "spread fingers to enlarge" but on facebook it deosen't work
many thanks in advance