What size plate do you want? Semiconducting and plasmonic copper phosphide platelets with sizes tunable from the nanometer to the micrometer scale have been developed, and their photoresponse and photovoltaic activity studied. The size control was achieved by controlling the nucleation density of materials in the reaction system. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.