hi sebastienmarion,

the class you are looking for would be TextEntryBox
However this will effect all other Text entry boxes as well in your survey
You could style the text boxes for this question only by using javaScript

for example like this:
Qualtrics.SurveyEngine.addOnload(function()
{
let teb = document.getElementsByClassName('TextEntryBox')

console.log(teb)

for(let i =0;i teb[i].style.width= "70%";
teb[i].style.marginLeft= "0.8em";
}
});
Best regards

Rudi