Kosel theme optimum size [SOLVED]

Image sizes and the theme width do not need to match exactly.
It is perhaps unlikely that images will be displayed at 100% anyway.
Showkase pages (and all its supported gallery types) are responsive and images are dynamically scaled (if necessary) so that they are displayed in their entirety respecting their aspect ratios and without cropping.
The image for the 'About' page, for example, is scaled to fill half the width of the page and images within Juicebox gallery pages are scaled to fit within the gallery's image area. (The images will not be displayed at their actual 1024px width.)
If you have really large images and wanted to display them much larger than the default theme width will allow, then you could certainly increase the theme width.
However, for your own scenario, I would personally just leave things as they are and allow Showkase (and the galleries) to dynamically resize the images if necessary.
You can, of course, try a few different settings for your theme width and settle on the one that you think works best for you and your images.

For other users reading this thread, the FAQ that md.michel is referring to is this one:
What is the optimum image size for the Boma and Kosel themes?