Instead of being forced to use (or modify) the standard "Medium" and "Large" image size post insert options, use the following code in your functions.php to allow additional image sizes to the WordPress drop down menu.
// Set the new image sizes
if(function_exists( 'add_image_size')){
add_image_size('blog-large', 900, 700, false);
}
// Tell the media panel to add the new size to the dropbown
function custom_image_sizes($sizes) {
// add more sizes to the array below if needed
$addsizes = array(
"blog-large" => __("X-Large")
);
$newsizes = array_merge($sizes, $addsizes);
return $newsizes;
}
add_filter('image_size_names_choose', 'custom_image_sizes');