a css rule
.containerClass .ajax__html_editor_extender_container
{
width: 100% !important;/*important is really important at here*/
}
and a surrounding div, fixed problem.
<div class="containerClass">
<asp:TextBox ID="tbEditor" runat="server"
TextMode="MultiLine" Rows="10" style="width:100%">
</asp:TextBox>
<asp:HtmlEditorExtender ID="tbEditor_HtmlEditorExtender" runat="server"
Enabled="True" TargetControlID="tbEditor">
</asp:HtmlEditorExtender>
</div>
fixed my problem.hope this helps someone too.