There are no truly miniature species of Zygopetalum.
Zygopetalum triste is the smallest one that you are likely to find, narrow upright foliage maybe staying under 8"-10" tall, and spike a bit taller. It can be intensely colored but not usually dramatically marked, though some clones may be. It is easier to grow than some because it tolerates warmer temperatures.
Zygopetalum microphytum may be a bit smaller, but probably much harder to find. It is a cool grower, with spotted petals but muted colors.
Zygopetalum sincorana is another small warmth tolerant species, but with a taller spike to 16". It will also be hard to find. The flowers can be dramatically spotted and streaked, but also without intense coloring.
If you aren't completely opposed to hybrids (like Zygonisia) you might also consider Propetalum (Promenea x Zygopetalum) crosses. They will be smaller than typical Zygos, mostly with spotted yellow flowers.