Question

I don’t want set this

the_post_thumbnail( array(100,100) ); 

because it output me a fixed size image, but I want obtain max image size, obviously with correct aspect ratio.

Answer 1

I believe you want this:

<?php the_post_thumbnail( "full" ); ?>

In addition to specifying a size, there are also multiple keywords such as “thumbnail”, “medium”, “large” and “full” which get you different premade sizes. You can also register your own image size using

<?php add_image_size( $name, $width, $height, $crop ); ?>

Then every time you upload an image it will crop to that size as well