Limit WordPress media uploader maximum upload file size

functions.php

<?php

/**
 * Limit WordPress media uploader maximum upload file size
 * Uploading very large images is pointless as they will hardly ever be used at full size.
 * Crunching larger files takes more memory; larger files take more space too.
 *
 * @param	mixed	$file	the uploaded file item to filter
 * 
 * @return 	array 	$file	the filtered file item with response
 */
function limit_upload_file_size( $file ) {
	
	$images = 1024; // size in KB (example)
	$others = 2048; // sizee in KB (example)

	// exclude admins
	if ( ! current_user_can( 'manage_options' ) ) :

		// get filesize of upload
		$size = $file['size'];
		$size = $size / 1024; // Calculate down to KB

		// get imagetype of upload
		$type = $file['type'];
		$is_image = strpos( $type, 'image' );

		// set sizelimit in kB
		$image_limit = $images;
		$others_limit = $others;

		if ( $is_image == true && $size > $image_limit ) {
			$file['error'] = sprintf( __( 'WARNING: You should not upload images larger than %d KB. Please reduce the image file size and try again.' ), $images );
		} elseif ( $is_image == false && $size > $others_limit ) {
			$file['error'] = sprintf( __( 'WARNING: You should not upload files larger than %d KB. Please reduce the file size and try again.' ), $others );
		}

	endif;

	return $file;
	
}
add_filter ( 'wp_handle_upload_prefilter', 'limit_upload_file_size', 10, 1 );