Any way to hide a blog posts text if it’s longer than let’s say 300 characters

You could use this filter which can be found in the default WordPress themes functions.php file (twentyten). This will limit the post to 300 characters, so theres no need to hide it.

function twentyten_excerpt_length( $length ) {
    return 300;
}
add_filter( 'excerpt_length', 'twentyten_excerpt_length' );

There are other means to achieve the same thing, but stick to whats already available and you should be fine.