Index: wp-content/themes/twentyeleven/editor-style.css
===================================================================
--- a/wp-content/themes/twentyeleven/editor-style.css
+++ b/wp-content/themes/twentyeleven/editor-style.css
@@ -4,3 +4,8 @@
*/
+
+ html .mceContentBody {
+ 	max-width: 584px;
+ }
+ 

* {
Index: wp-content/themes/twentyeleven/editor-style-rtl.css
===================================================================
--- a/wp-content/themes/twentyeleven/editor-style-rtl.css
+++ b/wp-content/themes/twentyeleven/editor-style-rtl.css
@@ -10,2 +10,3 @@
	float:right;
+ 	max-width: 584px;
}