Image size in WP Gallery

How do I get images to show up in full size on WordPress gallery. Images should be in proportion and fill up the post

After reading on Google, this is what I have tried...

I have set all size to 0 in setting > Media and uncheck "Crop thumbnail to..."

I have also added following code to the themes function.php

function wpmayor_filter_image_sizes( $sizes) {

unset( $sizes['medium']);
unset( $sizes['large']);
unset( $sizes['wysija-newsletters-max']);

return $sizes;
add_filter('intermediate_image_sizes_advanced', 'wpmayor_filter_image_sizes');

function wpmayor_custom_image_sizes($sizes) {
$myimgsizes = array(
"image-in-post" => __( "Image in Post" ),
"full" => __( "Original size" )
return $myimgsizes;
add_filter('image_size_names_choose', 'wpmayor_custom_image_sizes');