Insert photo from Camera to e-mail - show size choice on iPhone
Unfortunately, as I looked through Settings, as I thought, I couldn't find a way to change a settings like that. To be honest, I'm surprised that you had the option to remember the setting, as I have never had that (and still can't duplicate that).

I would recommend rebooting the phone to see if that, for some reason, fixes the problem. Also, as I'm sure you are well aware of, it should be mentioned that when you Message (MMS/iMessage) a picture or video it won't ask you the size, and when you email a video, it won't ask you the size.

If you really wanted to, you could reset the phone's settings (Settings > General > Reset (at the bottom of the list) > Reset All Settings). Honestly, I've only done the wipe all content and settings, so I'm not exactly sure what all it will reset, but it should take care of that problem. Don't forget to back up just in case.