I tried having only the Large selected (2048x1536)...
Showkase will always generate a medium size image in additional to the optional large and small size images.
You'll notice the Large and Small checkboxes read 'Large images' and 'Small images' respectively whereas the Medium checkbox reads 'Medium image resize'.
If, in the 'Multi-Size Images (Pro)' section, you have only the Large checkbox selected, then Showkase will generate a medium size image according to the settings in the upload module and a large size image according to the settings in the 'Multi-Size Images (Pro)' section.
When Juicebox displays the gallery, the medium size image will be used for the regular gallery display and the large size image will be used when the image is opened in a new window or downloaded.
You can check out the logic for which images are used in a multi-size image gallery here.
If you deselect all three checkboxes in the 'Multi-Size Images (Pro)' section ('Large images', 'Medium image resize' and 'Small images'), then Showkase will generate just one image for the gallery (according to the settings in the upload module) and, as there are no other image sizes available, Juicebox-Pro will have no option but to display this image in the gallery (and also use it for when the image is opened in a new window or downloaded).
Incidentally, it is always the medium size image dimensions that are displayed on the 'Images' tab.
I hope this helps to further explain how things work.