If the pictures are in your camera roll, here are the directions to choose the size of the photos when emailing them. Go to the camera roll and tap on the desired photo, then tap on the next button at the top of the screen. On the next page, tap on the email icon, then select the account if you have more than one. On the next screen, tap on the word images. This brings up a choice of sizes. Select the one you want and tap on it, then fill out the rest of the information on the form and send it as you would any other email.
Here are some photos from my iPad which should illustrate the method quite well with the exception of the second photo. This is where, on the iPhone, you tap a next button at the top right.