body#tinymce.wp-editor { 
font-family: Arial, Helvetica, sans-serif; 
margin: 10px;
max-width: 1000px !important;
}
body#tinymce.wp-editor-container {
max-width: 1000px !important;
}
body#tinymce.postarea {
max-width: 1000px !important;
}
body#tinymce.wp-editor-expand {
max-width: 1000px !important;
}