Just a quick update to let you know that you can now edit the size of the thumbnails even when a collection already has entries.
If you go to the edit collection page, you will notice the size drop down menu.
Just a note here, since the actual size of the older thumbnails does not actually change, if you select a much larger size (eg from 50 to 200px) you will notice a blurring of the already present thumbnails. This is due to the browser rescale to fit the new size. Of course, this will not happen to any entry added from there on and it will not happen when making the thumbnails smaller anyway.