Thanks Sebastian for your reply, but I found a better way, and would like to share it.
1. Install dnnWerk.RadEditorProvider, one can customize the toolbox and other features, per user role.
2. Open the ConfigDocumented.xml in the Telerik folder \Providers\HtmlEditorProviders\Telerik with full reference to all the additional options.
3. Copy
<property name="MaxTextLength">xxxxx</property>
and paste it in the config file of the user group and portal number you have customized it is here \DesktopModules\dnnWerk.RadEditorProvider\ConfigFil
In my case it is ConfigFile.Registered.PortalId.0.xml
xxxx state the max length of the text (integer)
I tried it and it works, I get popup dialog box with the error “1000 . . “ 1000 is the the value of the max length of the text one has selected.
I can not find a way to place my own error message saying "The max length permitted is 1000 characters, please go back and reduce the text length"”
From the Repository module, I know this is implemented by the resx file, but I need to place the reference in the resx file and insert my custom error message.