In Jain tradition, there is no single universally accepted “smallest” Chaityavandan sutra.

If you mean the shortest commonly used prayer in Chaityavandan, the most basic and universal one is:

Navकार Mantra / Namokar Mantra

णमो अरिहंताणं णमो सिद्धाणं णमो आयरियाणं णमो उवज्झायाणं णमो लोए सव्वसाहूणं

Its meaning is the salutation to the Arihants, Siddhas, Acharyas, Upadhyayas, and all Sadhus.

If you want, I can also give you the shortest full Chaityavandan form used in practice.