Question: what is the smallest carnivous animal at a fully grown stage?

Depends on how you define an animal?

The worlds smallest fish is less than 10mm long, but is likely to be an omnivore (plant and meat eater) rather a true carnivore. Some gobies measuring 25-30mm are likely to be the smallest true carnivores, with a backbone.

A tiny chameleon measuring 30-40mm is the smallest reptile carnivore.

However many invertebrate species are are carnivores. Within these broad taxonomic groups, if I had to chose I would think id would be one of the mites. Their average size is between 0.5-3.0 mm, some even going down to less than 0.1mm in length. While many are parasitic others are also carnivorous.

So a mite would probably be my pick of the smallest carnivorous animal at fully grown size.