What is the optimal pixel size for images for a marketpress store?

Hi there, I am using multisite and marketpress. Some of my store sub sites upload a picture in their media gallery and when it comes up on the main global store page listing, the picture is huge and loading slow so I went in and saw that the size was around 3000 or higher so I saved their picture to my computer, resized it to 500 and then after deleting the huge one, uploaded my resized one and it looked normal on the global product page. Now I am finding that some store owners' images when clicked on show up as a nice large image in the lightbox while other users' images are lit up but not much larger than the original on the store page. All store owners have the same settings of wp medium size checked in store settings presentation. Further investigation showed me that images uploaded at around 1200 or 1500 size are showing up as a nice large image when double clicked in the light box but still normal on the global shop page. What is the optimal pixel size for images so that they look uniform on the global product page and that they are enlarged in the lightbox?