My bookmarks open in some kind of quarter sized vertical screen (abt. 2" wide); how do I get it to full screen?

I click on bookmarks; the drop down menu opens; I click on a bookmark; it opens in less than a quarter screen occupying 2 vertical inches on the left side of my screen. I can scroll from the left to right via the task bar at the bottom of this screen but there are no visible icons for simply enlarging this screen. Very annoying!

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