It's the black, apple-shaped icon in the upper-left of your screen.;
,, It's in the upper-left of the preferences window.
 It's near the top of the dialog box.
 The selections are small, medium, or large, and are not customizable.
Click on the Apple menu.
Click on System Preferences.

,
Click on General.
Click on the "Sidebar Icon Size" drop-down menu.
Click on your preferred icon size.
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