The simplest fetch program you'll ever find, written in C. No seriously, this is what it looks like:
You can use the -i flag for base 1024 units (KiB, MiB, GiB, etc). By default the program uses base 1000 units like above (KB, MB, GB, etc).
I maintain a package for simplest-fetch on the AUR:
yay -S simplest-fetch-git
Alternatively you can just clone and run the build script, an executable will appear in a new build/ directory! For both of these the only platform I'm going to actively support is linux, chances are it'll work on BSD distros but I'm not sure, same with macOS.