WordPress | File size and extension

file_size_extension.php

<?php
$download_file = get_sub_field('download_file');
$download_url = $download_file['url'];
$download_path = pathinfo($download_url);
$download_ext = $download_path['extension'];
$download_size = filesize( get_attached_file( $download_file['id'] ) );

function format_bytes($size, $precision = 2) {
    $base = log($size, 1024);
    $suffixes = array('', 'KB', 'MB', 'GG', 'TB');
    return round(pow(1024, $base - floor($base)), $precision) .' '. $suffixes[floor($base)];
}

$download_size = format_bytes($download_size);
?>

Getting the file extension and file size of an uploaded file

Pretty self explanatory really. Uses PHP's filesize (more info here) though it returns bytes, the format_bytes function converts those bytes into a sensible format.

Stick the function in your theme's functions.php and use where you see fit.

Could be broken up into a number of functions to de-verbose. I'll leave that to you.

A more specific, versatile and recent file size function is available here