You need to add the Size (bytes) column in place of the current one.
Quickest way to do this is to right-click the column header and select General -> Size (bytes).
If you want to make the change permanent then you should apply the change to all of your Folder Formats. See the Folder Formats FAQ for more information.
You can also make a button which toggles the Size column between different modes for the current lister:
Set COLUMNSTOGGLE=sizeauto(1)
Set COLUMNSTOGGLE=size(1)
I use that because the "auto" column is usually best (for me) but sometimes I need to see the exact sizes instead.