Best way I can think of is to use a online image size calculator like this one. (Insert roughly what size you'd like in pixels then look down in the "file size" column to get a general idea for the various file types like JPEG.)
Once that's done, go into PS Touch and resize accordingly using the ampersand icon in the upper right, 2nd icon from the right > Image Size. Enter a width in pixels. (Height should automatically be adjusted to obtain proper ratio.) Then export as a file type such as JPEG.