Click on View menu. Select details. This will, among other things, give file sizes within the selected folder.
Click on the Tools menu next, then click Folder Options. Click on the View tab. Click on the "Apply to all Folders" button. Thats it all your Explorer folders will ipen with the dtails view.
I also use Powerdesk Pro by Ontrack. There is a free version, but I have paid. It is an enhanced Explorer with lots of extras including the ability to have a two pane view, i.e. two explorer windows open. Google powerdesk and you'll find lots of places to download it.