For those using the standalone mGBA, i suggest you give Enhanced mGBA a look. Latest commit was 24-11 but it works great, has screen size changing on the fly and an improved gui.
http://www.gc-forever.com/forums/viewtopic.php?f=13&p=33526
http://www.gc-forever.com/forums/viewtopic.php?f=13&p=33526