I've been testing with that, but there is something coded into Start11 that is overriding size for some icons.
If you pick image, and select DefaultMedium.png while it is in the Start11 default folder (C:\Program Files (x86)\Stardock\Start11\StartButtons), it will appear as a fairly small button (and not the correct color, meaning it is possibly not even loading that exact file but something coded into Start11)
If you copy DefaultMedium.png to any other folder it will load as a large icon taking up all the space of the start button area.
I was not able to create significant empty space or resize the icon with any amount of tinkering with the spacing of the icons from each other or amount of empty/transparent space around the icons. even tried filling in a faint area around the icon that is slightly translucent and Start11 still resizes it to ignore that area.
Update: If i make the icon absurdly small (20px) then I can pad it with space and it behaves as expected, but it looks terrible at that resolution.