[LifesGreatestGift]

This code

/* Quick Reply Editor */
#cke_contents_vB_Editor_QR_editor {
    height:300px!important;
}
/* Quick Reply Editor - END */

will actually affect both VM and Quick Reply if left in additional.css, If you only want to affect VM then open template member.css and add the following

/* Visitor Message Editor member.css */
#cke_contents_vB_Editor_QR_editor {
    height:300px!important;
}
/* Visitor Message Editor member.css - END */

--------------- Added [DATE]1308446298[/DATE] at [TIME]1308446298[/TIME] ---------------

Change height attribute to suit your needs.