Comet 17P/Holmes, 21.9 Nov 2007
From the image obtained, I measured the coma to be 34.0' wide (that is more than an apparent moon diameter) at 2007 Nov 21.90. This corresponds to 2.4 million km in reality.
Below is the image obtained. It is a stack of 29 images of 5 seconds exposure each, taken with my Canon Digital Ixus 400 pocket camera at maximum zoom (3x) on a fixed tripod. Below the image are the updated size diagrams.