What is the image size sweet-spot?

[julianadormon]

I have been crawling the discussions here hoping to find any recommended sizes for training images.

We are building an app that will capture multiple images of each person so we can control the input. The app will capture a full face and neck. So the face will occupy most of the image's available space. Can anyone recommend the smallest sized file we should make to keep processing times to a minimum?

For instance is 400px x 500px suitable at 72dpi? What would be suitable jpeg quality settings as a percentage?

Thanks in advance.