Change the style of h1, h2, only in post entry

by

You can change the styling of the editor content with your own CSS stylesheet via the add_editor_style function:

function wpa_add_editor_styles() {
    add_editor_style( 'custom-editor-style.css' );
}
add_action( 'init', 'wpa_add_editor_styles' );

Then in your stylesheet, add your rules:

body#tinymce.wp-editor h1 {
    font-size: 50%;
}