There are various ways. You could set it individually in the -> tools -> folder options, then in the "general" item in the drop down menu you can select Size (bytes) & add it to the right side, confirm & save the settings. For a more general approach, please have a look in the -> main preferences -> preferences -> folders -> folder formats -> file type formats (for example). There you could set the "standard" folder format to have Size (bytes) included, or for the other file type format as well.
You also could have a look in the status bar code sequences, just search for the keywords in your help file (F1). You can, for example, have the size in bytes for all your selected files & folders shown in yor status bar with the code {sb}.