I'm using a computer, but if you can do a Save, you should be able to change the size iin the dialog box that comes up. Mine has a slider bar (see below, it's the Quality slider bar). It's not the number of pixels; it's the number of Kilobytes. As you move the slider bar, it should show you the KB for where it's at. Stay below 1.5 MB and you should be good.
I can't just do a Save As with a saved file, so I do a Copy then File - New From Clipboard and then Save it using the slider bar to change the KB's.
Keep in mind, a .png file will use a LOT more KB's than a .jpg so if your screenshot is saving as a .png, just simply changing that setting will lower the number of KB's.
