Subject line:
#postform input.textbox {width: 100px; }
Comment box:
#postform textarea.textbox {width: 200px; }
If the extra text in the form area (Don't auto-format; check spelling; etc) still causes your layout to stretch, you'll need to edit the width of the box around the entire comment form:
form#postform {width: 300px; }
The OpenID URL line if the OpenID option is selected:
#postform input#oidurl {width: 100px; }
The username and password lines if [other] is selected under "Logged in user:"
#postform input#username {width: 100px; }
#postform input#password {width: 100px; }