You can set the number of vertices in a vertex array during construction (http://www.sfml-dev.org/documentation/2.3.2/classsf_1_1VertexArray.php#a4bb1c29a0e3354a035075899d84f02f9):
sf::VertexArray vertexArray(sf::Points, 100); // 100 vertices
and/or you can resize (http://www.sfml-dev.org/documentation/2.3.2/classsf_1_1VertexArray.php#a0c0fe239e8f9a54e64d3bbc96bf548c0) it later:
vertexArray.resize(200); // now 200 vertices