Hello Jens,

Thank you for the kind words regarding the Width of the TextArea, and I am happy to read that you are already using it!

What the rows HTML attribute does is to place a fixed height of the textarea (default HTML element). A row itself is a combination of font-size - the default for Google Chrome is 22px, padding, and margin which default to 0. If you have the rows attribute set to 10 the total height of the element would be 10 * 22 * the line-height or about 220-250 px. The height of the row is controlled by the line-height CSS property. To get a behavior similar to what you have using the rows attribute you can use the Class parameter exposed by the TelerikTextArea and cascade the height CSS property. Below, I have made an example that you can use as a base. 

<style>
    .custom-textarea-class {
        height: 250px;
    }
</style>

<TelerikTextArea @bind-Value="@TextAreaValue" Class="custom-textarea-class"></TelerikTextArea>

@code {
    public string TextAreaValue { get; set; }
}

That being said, we believe that this is a valid feature request.

Regards,
Svetoslav Dimitrov
Progress Telerik