What did you use to reduce it?

I would use Irfanview, a well-known free image viewer and editor.

First you would 'crop' the image to the approximate shape you want, and you have done that, but it would be better to use the original image and set the shape. Then you would resize it to 50X150, using the 'retain aspect ratio' setting changing the DPI (dots per inch) if necessary to reduce it to the 6.8kb.

Any standard image editor will have similar settings, maybe with slightly different names or locations.