A quick look through my Venetian beads found that most of the 12 mm diameter, cylindrical fancy beads are longer than yours, more like 18-22 mm. The few 12 x 15 mm beads I found were not exact cylinders - they were gently shaped at the ends into "barrel" shape.

The more typical size is 10 mm diameter for shorter, 15 mm long fancy cylindrical beads.

Note: by "fancy" I mean beads that are not decorated with millefiori slices.

Never thought about this before so an interesting question!

Some have speculated that there were 100s of thousands of different Venetian bead designs made, so it's impossible to know all of them. I'm sure a large percentage of the designs will never be found on a sample card.