Hi Loid,
My apologies for the late reply. You can make a font bigger by setting the CSS rule font-size that targets the relevant HTML nodes
Assuming you are using the native-IBEX AcceptabilityJudgment controller, which is composed of the FlashSentence controller to render the s parameter and the Question controller to render q and as, you can edit the file FlashSentence.css in your project’s Aesthetics folder to modify the font size of the s parameter by adding:
p.FlashSentence {
    font-size: 3em;
}
To change the font size of the q and as parameters, you can edit the file Question.css and add: 
p.question-text {
    font-size: 4em;
}
li.normal-answer {
    font-size: 2em;
}
Note that those changes will affect any other item using the FlashSentence and/or the Question controller
Jeremy