“size” being the scale of the cube. 0.5 means that the cube has a length of 1 unit
// Front Face
ffpBegin(FFP_QUADS);
ffpVertex3f(-size, -size, size);
ffpVertex3f(size, -size, size);
ffpVertex3f(size, size, size);
ffpVertex3f(-size, size, size);
// Back Face
ffpVertex3f(-size, -size, -size);
ffpVertex3f(-size, size, -size);
ffpVertex3f(size, size, -size);
ffpVertex3f(size, -size, -size);
// Top Face
ffpVertex3f(-size, size, -size);
ffpVertex3f(-size, size, size);
ffpVertex3f(size, size, size);
ffpVertex3f(size, size, -size);
// Bottom Face
ffpVertex3f(-size, -size, -size);
ffpVertex3f(size, -size, -size);
ffpVertex3f(size, -size, size);
ffpVertex3f(-size, -size, size);
// Right face
ffpVertex3f(size, -size, -size);
ffpVertex3f(size, size, -size);
ffpVertex3f(size, size, size);
ffpVertex3f(size, -size, size);
// Left Face
ffpVertex3f(-size, -size, -size);
ffpVertex3f(-size, -size, size);
ffpVertex3f(-size, size, size);
ffpVertex3f(-size, size, -size);