To adjust the size of vertices in the Blender viewport,go to on Main menu - File - User Preferences - Themes - 3D Viewport and scroll up to this setting named Vertex Size.
See the next screenshot with this option:

2D, 3D, game, games, online game, game development, game engine, programming, OpenGL, Open AI, math, graphics, design, graphic, graphics, game development, game engine, programming, web development, web art, web graphic, arts, tutorial, tutorials,