how to find objects dimension in image??

You can get the image Width and Height using .NET Image Class as follows:
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System.Drawing.Image _img = System.Drawing.Image.FromFile(@"C:\temp\Image1.TIF");
MessageBox.Show("Image Width:" + _img.Width.ToString() + Environment.NewLine + "Image Height:" + _img.Height.ToString());
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Note that not all file formats are natively supported under .NET, so you may consider using a professional toolkit like LEADTOOLS, which is also easy to use. The following simple code shows how you can get the dimensions of an image:
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RasterCodecs _codecs = new RasterCodecs();
CodecsImageInfo information = _codecs.GetInformation(@"C:\temp\Image1.TIF", true);
MessageBox.Show("Image Width:" + information.Width.ToString() + Environment.NewLine + "Image Height:" + information.Height.ToString());
/*************************/