When using the rich text column type in SharePoint lists the edit control has a fixed width of 384 pixels. This is often too small for easy editing.
If you need to change the width for a single form, open the form in SharePoint Designer and add a style defining ms-rtelong at the top of the content place holder in which the form is displayed with new dimensions, e.g.
<asp:Content ContentPlaceHolderId=”PlaceHolderMain” runat=”server”>
<style type=”text/css”>
.ms-rtelong {
width:700px;
height:600px;
}
</style>