You can only resize it form left to right. Place your mouse pointer on the bar on the right of 'GO' or to the left of 'address' and when it changes shape left-click, hold and drag to the right for go and left for address.
Firstly, right-click on a blank space on a toolbar, and make sure there is not a tick next to Lock Toolbars. If there is, untick it.
Then, try to hover over the thin dotted line at the end of any collection of buttons that are on the same row as your address bar (the cursor changes to a double-headed horizontal arrow), then drag those buttons up or down, to leave the address bar on a row of it's own. This will give the maximum space for the address bar, and it should automatically expand to fill the available width.
If it does not expand, then follow the advice from GANDALF <|:-)>
If it all goes horribly wrong, just right-click a blank space on a toolbar, choose Customize, then Reset.