image resizing on post content without css and html tags

You should look at the following: https://developer.wordpress.org/reference/functions/add_image_size/

eg add this to your theme’s functions.php or plugin:

add_image_size( 'newsletter-image', 800, 600 ); // 800 pixels wide by 600 pixels tall, soft proportional crop mode

Then use the image like this:

if ( has_post_thumbnail() ) { 
    the_post_thumbnail( 'newsletter-image' ); 
}