fcvolunteer on "[Plugin: Contact Form 7] element-resizing does not work"

It seems like the existing css is "overruling" the shortcode method. You've already assigned classes to your fields you can use those to modify the widths. See example:

.your-name input {
    width: 200px;
}
.your-email input {
    width: 200px;
}
.your-phone input {
    width: 200px;
}
.your-message textarea {
    width: 200px;
}
.quiz-kontakt input {
    width: 50px;
}