That will depend on what film we are recording to. Here are the values:
35mm slides and negatives: 4096 x 2732 pixels
6x7cm transparencies and negatives: 8192 x 6702 pixels
5″x4″ transparencies and negatives: 8192 x 6732 pixels
Retro-View reels: 1390 x 1260 pixels
Note that these are the actual values we record at and therefore optimal sharpness will be obtained – if your images exceed this that is fine. If they are less that these values then the further away from optimal, the less sharp the slide will be. Whether this matters depends on variables such at size of projected image, distance audience are from the screen, quality of projection lenses, amount of detail in the subject matter, and customer expectations.
We can produce slides from any image, no matter how small. But the higher resolution the original image, the sharper the slide will be.
Note that there is no point using Photoshop or similar programs to resize the files to make them ‘bigger’. It won’t improve quality – it is the original image quality that matters.