How big?
Long-side size in mm. Around 100 suits a small night light on a desk; 40 for a keyring; 150 or more for the wall.
Actual-size reference: the grey shape is a credit card (85.6 × 54 mm).
How bright, how much contrast?
A lithophane works by thickness letting light through. Stronger contrast gives darker shadows and more depth but needs a brighter light; the soft version is clear with a normal desk lamp or at a window.