Other factor (and a wild card) determining how various sized organisms fare with temperature changes is metabolism. As far as something like a metal statue cooling off, it is mass vs surface area as prior answers note.
This assumes homogeneity: a bronze statue. If the layer interfacing with the outside is a poor conductor of heat then this will slow conduction of heat from the core to the outside air or water. Blubber is an insulator. Feathers and fur are insulators. I am pretty sure fairies wear flesh-toned neoprene wetsuits; also an insulator.
Metabolic activity is essentially burning carbon compounds inside the body. In cold conditions a warm blooded animal can increase the metabolic rate to produce more heat. Human (usually babies but it can persist to adulthood) have special fat called brown fat with loads of mitochondria: the purpose is to make heat.
Cold blooded animals like tuna have other tricks by which they use increased metabolism to produce heat. The problem for very small creatures like hummingbirds is limited body space makes it hard to carry so much metabolic fuel on board, especially as carbohydrate. I propose that to overcome this problem your fairies can subsist largely on vodka and butter.