Hi Eveline,
You do not say which type of questions your quizzes consist of. Anyway, the following workaround works for MCQ questions.
If you are using the formal white theme, in file fw_fonts.css, at the end, add this line:
#mod-quiz-attempt table table table table td {font-size:2em;}The result is shown in attached screen shot. Of course you can use any font-size unit and size you like. I am using ems here because I prefer relative units rather than fixed ones.
I hope that helps,
Joseph
PS the screen shot comes from my own orange chocolate theme, where quiz answers are left-aligned rather than right-aligned as in standard Moodle themes. BTW, right-alignement is something which many Moodle users seem to be against. Let's hope it disappears from version 1.6 altogether!