Really nice job on this. I really enjoyed the color scheme and your art style.
As far as reducing the file is concerned here's some help for you
1) Open it in MS paint
2) Go to the image menu
3) Select resize/skew
4) Then adjust the vertical and horizontal setting. Let's say you want it to be 1/2 the original size. So type 50 in both of the fields, hit ok, and you're done.