Hi,
You can add custom validation for any PR field in registration form using PR action/hook “pieregister_registration_validation_after”. Add something like following in your theme’s functions.php file.
function validate_profile_size($posts, $errors) {
// inspect element to check your correct profile_pic field name.
if( $_FILES['profile_pic_8']['size'] > 1000 ) {
$errors->add( "custom_pic_size" , '<strong>'.ucwords('error').'</strong>: '.apply_filters("piereg_Invalid_Username","'File Size is not valid.'"));
}
}
add_action('pieregister_registration_validation_after','validate_profile_size', 10, 2);
Hope it helps.
Thanks