People use the Windows 10 start menu in different ways. Some will pin numerous tiles and make maximum use of a push button drawer of screen real estate. Others will put absolutely nothing on their start menu and only use the applications list like it’s 1998. In either boat start menu size matters.
The below GIF shows how easy it is to resize your start menu to fit your needs. I like my start menu at work to be about a quarter of my screen, but at home it is thinner.
The start menu width can be as wide as your screen, if you have tiles to occupy that space. The height is limited, but is nearly the full height of the screen.
You can also set the Start Menu to use Full Screen mode which was introduced in Windows 8. To toggle this, and other settings, you would open Settings -> Personalization -> Start.