Definition of images

There are three definitions when it comes to image sizes.
the first defines the thumbnail size
the second defines the in product page image size
the third defines the in product page image on klick(zoom)size


1. first of all, are these statements correct ?

2. If so, how can someone define only one value, like height, for example 300 pixel and let the system calculate the porpotional width to it? Can it be done by just leaving the width empty?

3. And ... what if you want to define max length and width? Like for example you have images with different orientations. So for all horizontal images you want a max width and an automated height and for vertical images max width and let the system calculate the poportional height for it.

thank you

but if I define 500x600 because i want max 500 width and max 600 height and my image is 600x800, wont if shrink to 500x600 and make it stretched in a bad way ?

I assume it will keep the largest set in settings if both are larger or smaller than both values

thank you very much