You can do some basic changes without Power User mode. Just go into Settings (Alt P), and under Start Page, there are some Speed Dial settings. Like for instance, you can set the maximum number of columns. That gives you the number of speed dial entries in a row. You can also use big Speed Dial thumbnails or small ones as a setting. If you go into Power User mode, you get a few more settings, including being able to hide the Google Search bar.
Here's something on enabling Power User Mode. http://newsblog.wti.com/index.php/2013/11/opera-19-developer-update-how-to-enable-operas-power-user-mode/
Remember thought to first click outside the search box with your mouse (not mentioned in the blog post on the link above), before using the arrow keys per the instruction: Up Up Down Down Left Right Left Right Left Right b a.
After you do this, and enable power user mode, you'll have a few more settings, though perhaps, the settings already in Alt P without this, under Start Page, will be sufficient for you.