The best app i've found (but I'm a longtime Photoshop/Lightroom user so like to muck about with photos LOL) is Filterstorm. Next best is Photogene but if you just need to do a minor bit the Photo Express would be a good choice
When you email from Photo app you will actually have a choice of small, medium or large files. Look in the top right of the email itself after you select your image(s) and choose email and touch where it says image size.
Diane