We are working on fixing this issue but unfortunately, I am unable to provide a firm estimate on when it will be available. The problem occurs if the RadEditor's toolbar calculates it size before the CSS resource files are loaded.
Could you please register the external RadEditor skins as it is shown in this KB article: Registering an external skin of RadEditor
You can also take advantage of the recently introduced CDN
support which will allow you to load the controls skins from the Telerik CDN. Telerik CDN is hosted on the Amazon CloudFront service.
You can find more information about our CDN support here
By loading the external or CDN skins you will ensure that the editor will load its skin.
Another possible workaround is setting width:100
% !important to the toolbar CSS selector, e.g.:
If you have multiple instances of RadEditor on the page you can set CssClass to the editor in the RadGrid's editform to use it for cascading.
the Telerik team