At HTML/CSS Inserts -> CSS Inserts, add, for instance:
HTML Code:
div#respond {
font-size: 0.8em;
}
h3.reply {
font-size: 1.2em;
}
textarea#comment {
width: 400px;
}
The first style is for the overall text around the comment form, the second for the "Leave a reply" headline, the third for the textarea width. You can change the X.Xem values (Xem, X.Xem, X.XXem) , and the textarea width can also be a percent value XX%.