In -> tools -> folder options -> columns you can chose different types of "size" like "size (Bytes)" or "size (KB)". After saving this will be available for the chosen folder, resp. the subfolders, depending on how you saved that setting. If you want to apply it to some "standard" folders, however, you can also have a look in -> settings -> preferences -> folder -> folder formats, where different types of special formats can be set. Please check also this FAQ discussing some more folder formats details: