on 08-09-2012 09:08 AM
The only measurement you should use is pixels because that is the only measurement that browsers understand.
The minimum photo size next Fall will be 500 pixels on the long side, but that will not trigger supersize and zoom. You will need a photo that is at least 501 pixels on the long side to trigger those features.
eBay recommends that your photo be at least 1600 pixels on the long side for best zooming ability. If the long side of your photo falls below 1000 pixels, you may see a message during upload that your photo is "not optimal". You will also see that message if your photo is compressed more than 10%, as eBay would like to see 90% jpeg quality.
Whether or not your photo appears the same size in the preview and enlargement is somewhat related to the number of pixels on your monitor. Larger screen sizes will start with a larger preview, so the enlargement may not enlarge at all.
But in general, if your photo is more than 640 pixels on the long side, you should see some enlargement. I see that most of your photos are too small to enlarge and the ones that do enlarge are displayed at 640 pixels.
However, when I look at the source photo for this image, I see that you have uploaded a full-sized 1600 pixel image. It may not be supersizing because it was uploaded prior to July or because the Picture Pack option was turned off during upload. Since you have a store, you should have gotten supersize all along, so I am slightly puzzled.
Did you use a listing service or file exchange to upload? If so, you may need to enable Picture Pack there.