Changing gallery size in visual editor

Hi there,

is there any way to select the gallery size in the visual editor? Like for example you would add size=”medium” to the gallery shortcode in the text editor, but that unfortunately isn’t an option. Would anyone be able to send me to the right PHP file so I can add that functionality myself? Have searched quite a bit but haven’t found anything, admittedly I haven’t done too much customization in the admin area before so any help would be greatly appreciated.