More often than not, this is labeled as "Hard drive." It could also be your (C:) drive, however. " If it is highlighted, you can also press Command + I (uppercase "i") to open the info pane. Once you've opened info, the amount of space should be visible as a number of Gigabytes (GB). This is the amount of space you have for files like music, movies, photos, and documents.

How would you rephrase that briefly using Chinese?
打开Finder并定位到硬盘驱。 按住“Control键”，同时点击驱动器，并选择“获取信息”。 查看硬盘大小和剩余空间。