Sample code
<obout:ImageZoom ID
="ImageZoom1" runat="server" BigImageSize="Custom" BigImageWidth="500" BigImageHeight="500" ...
></obout:ImageZoom>
c#
ImageZoom1.BigImageSize = OboutInc.ImageZoom.Enums.BigImageSize.Custom;
ImageZoom1.BigImageWidth = 500;
ImageZoom1.BigImageHeight = 500;