Apple - Mouse Pointer Locator

In the release of OS X El Capitan, just shake the cursor to locate it.

From Apple:

Call out your cursor.

Lost your cursor on your crowded desktop? Just shake your finger back and forth on the trackpad or give your mouse a shake, and the cursor gets bigger so it’s easy to spot.

To enable/disable this feature, go to System Preferences > Accessibility > Shake mouse pointer to locate

Shake mouse pointer to locate

Quickly move the mouse pointer back and forth to make it bigger.

Pointer Finder

This app will put a circle around your cursor when you press a key.

With OS X El Capitan and later, all you need to do is move the mouse back and forth quickly, and it will automatically make the mouse bigger. The advantage of this is that you don't need to manually set your mouse to a bigger size if you don't want to.

If this doesn't work for you, the option to turn it on can be found in System Preferences > Accessibility > Display > Shake mouse pointer to locate.