A study in Nagoya, Japan suggests the possibility that significant "park cool island" (PCI) impacts (i.e. parks that are at least 1-2°C cooler than their urban surroundings) only exist when parks are larger than a certain threshold (2 ha in the study). The study also shows that the intensity of PCI is mainly determined by areas of trees and shrubs in the park as well as the park's compactness, with irregular and belt-shape parks tending to have low PCI intensity.
Cao, Xin, Akio Onishi, Jin Chen, and Hidefumi Imura. (2010). Quantifying the cool island intensity of urban parks using ASTER and IKONOS data. Landscape and Urban Planning, 96(4), 224-231.