De acordo com Azeez Nadeem, um funcionário de suporte da Microsoft:
This cannot be changed. Windows Explorer will display the size column in the smallest unit, “KB” for simplicity. If you select your file and view the details pane, you will see the file in its true form, whether it’s MB or GB. There are third party programs that are able to do what you are asking [...]
Você pode tentar o Total Commander ou um gerenciador de arquivos semelhante.