The AcceptsReturn property is designed to restrict the input of the user when (s)he actually presses the return key. If (s)he pastes any multiline text it is up to you to customize the pasted text and to control the visualization of the RadMaskedTextInput. You can do this by subscribing to the ValueChanged event of the control. In its handler you will be able to detect if there are any new lines and returns and you will be able to change the Value of the component.
Regarding your second requirement you can try setting fixed height of the control to restrict it from resizing.
Pavel R. Pavlov
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