As you are probably aware, Microsoft has brought back the Start Menu with Windows 10. And while a lot of people joke that it took them this long to merely go back to Windows 7, there are UI improvements in this new Start Menu. The new and improved Start Menu allows you to control how it looks and how it fits inside your workflow.
The biggest control you have over it is the size. In Windows 7, you were given the same menu, inconsiderate of your needs. For example, I rarely actually do any clicking. I usually press the windows key (to toggle the Start Menu), then type away (it automatically searches), and press enter when it finds what I am looking for. This is why I like to keep my start menu as compact as possible, so I can continue monitoring my screen.
You can directly resize the height of the Start Menu. There is a lower limit for the height (the lead image shows what my resolution is restricted to) as well as an upper limit that stops the menu from quite reaching the top of your desktop. To resize it, simply take your mouse and hover over the top edge, just as you would on any other program window. Once your mouse changes into a double arrow, click and drag up or down. The width of the Start Menu will automatically adjust as you change the height based on the number of apps you have pinned, you cannot change it directly.
You can indirectly adjust the width by adjusting the number of apps in the Modern UI part of the menu (right click an app, select pin/unpin to start). Further, you can resize the pinned apps to adjust how much space they take. The start menu will only enable scrolling when the apps overflow, so the width can expand to the edge of the screen.
Since the Start Menu basically includes a miniature Start Screen, you can arrange and resize the icons as you would on Windows 8 to suit your workflow. Also, the theme of the Start menu is the same as your windows theme (in 'Personalize'). You can't, however, have the background the same as your desktop as you can with the Start Screen. You can see my experimentation with a hot pink theme in the title picture for yourself.
If you have moved on from the Start Menu and no amount of prettiness will make you take her back, you have the option to retain the Start Screen (click here to find out how).Further reading: Microsoft, Start Menu, Windows 10