Fight $SIZE as a global variable

From a785d3e06f4c27f4ac77227d0cf818a29324fe1c Mon Sep 17 00:00:00 2001
From: Jan-Benedict Glaw <jbglaw@getslash.de>
Date: Fri, 15 Mar 2024 21:56:45 +0100
Subject: [PATCH] get_filesize: Phase out usage of global $SIZE variable

[ edited by Mattia ]
Retain some previous stylistic choices.
Prevent SC2155.

Signed-off-by: Jan-Benedict Glaw <jbglaw@lug-owl.de>
Signed-off-by: Mattia Rizzolo <mattia@debian.org>