It's really easy to set the display size of a multi-line textbox in .NET once you know the correct properties to set. The multi-line textbox is equivalent to an <HTML> textarea.
To control the number of lines that display to the user you can set the "rows" property and to control the display width you can set the "width" property. You can also set the CssStyle of the textbox to control the width.
Here's an example:
<asp:TextBox ID="txtYourTextBox" runat="server" rows="6" TextMode="MultiLine" Width="450px" Text="Your text"></asp:TextBox>