Thank you for your post.
The file size is a bit misleading, as two pictures, both being the same size, can have much different file sizes.
The bottom of page 19 of the FileMaker Go Technical Brief (http://www.filemaker.com/products/filemaker-go/docs/techbrief_filemaker_go_11_en.pdf) displays the screen size for the iPod touch for portrait and landscape modes, with and without the Toolbar. For example, if you plan to have the Toolbar visible and the layout is to be displayed in portrait, then the screen size is 320 pixels in width by 385 pixels in height. Another factor to consider is the resolution. If you are using this primarily as a learning tool, then resolution may not be paramount, so 72 dpi may be sufficient. You will have to experiment to see what settings work best for you.
Let me know if you need additional clarification.