Questionnaire styles.css has the following code:

div.felement.ftextarea textarea {
width:600px;
}

This could affect every textarea in every Moodle form across the system (eg wiki annotations), yet I cannot find it being used in any page in the questionnaire.

To test I have downloaded a head version of questionnaire from cvs, and added this to a head version of Moodle (2.2dev) from git on my local pc in an environment set up using default settings. I then added a marker to styles.css (line 84) above like this:
div.felement.ftextarea textarea {
width:600px;
border: 2px solid red;
}
Then I set up a questionnaire with one of each type of question. I also tested changing the course format from the default weekly to topics, and tried changing the theme from standard to formal white. I also tried changing the text editor from Tiny to plain text area. Finally I tested removing this style altogether.

It does seem that this style is not used in questionnaire, but can affect other non-questionnaire pages (specifically some of our OU custom pages), so please could this style be removed from head (and possibly MOODLE_20_STABLE branch).