Evolution of the size distribution of InAs islands is examined via in situ ultrahigh vacuum atomic force microscope for samples grown via molecular beam epitaxy with InAs deposition at and just above the critical value for two- to three-dimensional growth mode transition. An initial independent island formation and growth followed by a global tendency towards island lateral size equalization is observed. The strain fields induced by the InAs islands in the subsequently grown GaAs cap layer are exploited to fabricate vertically self-organized multiple sets of islands. Optical properties of such vertically self-organized quantum box islands are examined via photoluminescence spectroscopy. Microtwins of very low linear density (<1/mu m), originating from the edge of the topmost set of islands are found for the samples with multiple sets of islands and are shown to be the type of defect that appears first to relax the accumulated strain around the multiply stacked islands.