[Plugin:Faster Image Insert]Custom image size

Would it be possible to add custom image sizes? The current available sizes for adding images are:
Thumbnail (150×150)
Medium (300×225)
Large (1024×768)
Full size

Instead, I would like to show most pictures at around 640×480 as Medium is just too small and Large is too big. Is there any way to make the pictures size to 640×480 or something alike? Or is the only way to achieve this, saving them as Large and then shrinking them manually?

Thanks!