I'm not going set this

the_post_thumbnail( array(100,100) ); 

since it output us a fixed size image, however i want obtain max image size, clearly with correct aspect ratio.

In my opinion you would like this:

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

Additionally to indicating a size, you will find also multiple key phrases for example "thumbnail", "medium", "large" and "full" which enable you to get different pre-made dimensions. You may also register your personal image size using

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

Then any time you upload a picture it'll crop to that particular size too