As suggested by its origin islette, an Old Frenchdiminutive of "isle", use of the term implies small size, but little attention is given to drawing an upper limit on its applicability.
The World Landforms website says, "An islet landform is generally considered to be a rock or small island that has little vegetation and cannot sustain human habitation", and further that size may vary from a few square feet to several square miles, with no specific rule pertaining to size.
A Tahitian motu off the island of Raiatea at sunset
A rock, in the sense of a type of islet, is an uninhabited landform composed of exposed rocks, lying offshore, and having at most minimal vegetation, such as Albino Rock in the Palm Island group off Queensland, Australia.
Sea stack, a thin, vertical landform jutting out of a body of water.
Skerry, a small rocky island, usually defined to be too small for habitation, especially in Ireland.
Subsidiary islets, a more technical application, is applied to small land features isolated by water, lying off the shore of a larger island. Similarly, any emergent land in an atoll is also called an islet.
Tidal island, small islands (not always islets) which lie closely off the coast of a mainland or a much larger island, being connected to it (and thus becomes a peninsula/promontory) in low tide and isolated by a channel in high tide.
Clive Schofield (2012). "Islands or Rocks, Is that the Real Question? The Treatment of Islands in the Delimitation of Maritime Boundaries". In Myron H. Nordquist; John Norton Moore; Alfred H.A. Soons; Hak-So Kim (eds.). The Law of the Sea Convention: US Accession and Globalization. Martinus Nijhoff Publishers. pp. 322–340. ISBN978-90-04-20136-1.