If you are running a recent version of Xerte then you can add the 'Script' optional property to the page and copy this in to it:
Code:
$('#initText, #answerTxt, #modelTxt').attr('rows',1);
Rows is the number of lines in the text box. You could also change the width in a similar way:
Code:
$('#initText, #answerTxt, #modelTxt').width('50%');