Hi Author,
StlAPI::Write function in OCCT 6.8.0 uses BRepMesh_IncrementalMesh to compute triangulations. That class can not let you control the number of nodes on planes. A possible workaround could be in preparing the triangulations using some other meshing algorithm, for example, Express Mesh (http://www.opencascade.org/support/products/emesh/).
Best regards,
Commenter-1