Add these code to your theme function file.
add_image_size('mythumb', 350, 200, array( 'left', 'top' ));
// third parameter is for cropped option, you can change that as your preference.
the_post_thumbnail( 'mythumb' );// image with the new dimensions.
Important: you need to regenerate thumbnails to make this work. You can use plugin for that. For complete study, check official guide:
https://codex.wordpress.org/Function_Reference/add_image_size