Have you tried disabling back face culling?
If your sprite doesn't have the correct winding, and back face culling is enabled , then it will not be drawn.
Also, in OpenGL the Y coordinate goes from bottom to top, so make sure that you are positioning the sprite inside the view (unlike the projection matrix where point 0.0, 0.0, 0.0, is the center of the screen, on an orthographic projection, it is the left bottom corner).
Although you can change this when you build the matrix.
You can also try a bigger sprite, like with a size of (100, 100).
One last note. You should try the function Shogun posted to calculate your matrix. Somehow, and i might be wrong, but it doesn't seem like your matrix is being calculated like it should (again, it may be just me).
Don't know if this is your problem, but try it and see.