Set Height and Width of AJAX BalloonPopupExtender in ASP.Net

Refer this article

ASP.Net AJAX Balloon Popup Extender Example: Cloud Rectangle and Custom Oval Styles

and set the height and width in style

<cc1:BalloonPopupExtender ID="BalloonPopupExtender1" TargetControlID="TextBox1" UseShadow="true"
        DisplayOnFocus="true" Position="BottomRight" BalloonPopupControlID="Panel1" BalloonStyle="Custom"
        CustomCssUrl="~/Styles/Css.css" CustomClassName="oval small" runat="server" />