Adress bar size and extensions' icons
I use Opera 19 and Opera Next 20. In both cases a limited number of extensions icons are showed in the extensions bar (the rest of them are missing) and the size of the addressbar is not modifiable, it cannot shrink.
I think the only solution at the moment is go to Opera menu -> Extensions then tick the checkboxes to hide the ones you don't really need there (making room for others).
The number of visible extensions is determined by how wide your window is. This is configured automatically.
As rafaelluik says above, you can choose which ones to have visible and not from the Extension manager.