Hi Mayur,
Unfortunately I couldn't replicate the issue at my end. Please have a look into the sample code snippet which works fine at my end. The MaxLength property of RadTextBox will sets the maximum number of characters allowed in the text box. So if you are pasting a content which length is greater than MaxLength then RadTextBox will show only the allowed number of character. Please provide your full code if it doesn't help.
ASPX:
<
telerik:RadTextBox
ID
=
"RadTextBox1"
runat
=
"server"
TextMode
=
"MultiLine"
MaxLength
=
"10"
>
</
telerik:RadTextBox
>
Thanks,
Shinu.