Abstract Protected areas are critical to biodiversity conservation. Yet many protected areas around the world are very small, and population persistence can be compromised in small habitat patches, particularly for large species. But we do not know how small is too small for long-term population viability, or the degree to which habitat area effects vary with mammal body size, for most tropical species. Here, diversity and species occurrence were assessed in a small national park that has long been isolated from other forest patches. The two largest Bornean carnivores, the Sunda clouded leopard (Neofelis diardi) and sun bear (Helarctos malayanus), may be locally extinct, but 12 smaller carnivores appear to be persisting. The banded civet (Hemigalus derbyanus), the Malay civet (Viverra tangalunga) and the leopard cat (Prionailurus bengalensis) had relatively high occurrence rates and were found in all habitat types, though were more common in hill (banded civet) or lowland (common palm civet) forest (LF). Occurrence probabilities for all of these species and the common palm civet (Paradoxurus hermaphroditus) were lower in areas more accessible to humans. Detection rates and estimated diversity suggest that this park has a small carnivore assemblage equivalent to those of much larger areas. These are the first results demonstrating size-specific vulnerability to area effects in Southeast Asian carnivores.