There is actually a reason for the three different sizes pf pictures, and it has to do with what you intend to to with the photo after it is sent.
If you are sending a 'snapshot' to a friend to view on another smart phone or small format PC then small is fine. Essentially if it is intended to be viewed once or twice, then choose small. Works great for this use.
If the viewer is going to be use a PC or other large format device, then the medium format if great. It has plenty of pixels to satisfy the display requirements of all but the most sophisticated monitor. This size is also good for printing at the 4x6 or maybe even a 5x7 size.
If you intend to work with the photo with something like Photoshop, or if you intend to have the photo processed professionally (as compared to printing it on a 600 dpi photo printer), use the larger size picture.
I use my iPhone camera, a point and shoot camera and a SLR high end camera a lot. If I am just fooling around, I will use the iPhone or pocket camera. If I am doing some serious photography (weddings, portraits, wildlife, etc.), I use my trusty SLR.
I will admit that the quality of the iPhone camera and my point-and-shoot are blurring with the quality of the SLR, there is still a difference when you blow the picture up to 11x17 or larger. Also, I don't think a bride would accept any explanation for me showing up at her wedding to take photos with only my iPhone ;o)
