The default size of the mouse pointers in OS X should allow them to be viewable in most situations, but there could be some instances in which the cursor can be difficult to locate. For example, if you need to dim your screen, it might be difficult to find the pointer; and sometimes, optional pointers -- such as those for text input or crosshairs -- can become lost among the textures of images on the screen.
This might be especially true when using projectors and large-format displays, where the relatively small cursor can be difficult to locate.
While you can eventually find your cursor by moving it around on the screen or bottoming-out its movement against a corner of the screen as a reference location, there are other options available in OS X to help prevent you from losing it in the first place.
One classic option for pointer location is mouse-trails, where a small footprint of the cursor shape will follow it around on screen. Apple used to have such an option in the Mac OS, but no longer includes it (though third-party alternatives like HotMouse somewhat fill this gap); however, it does have an option for making the cursor bigger, which, if used even in slight moderation, can substantially affect its visibility.
To do this, you need to open the Accessibility system preferences, which can be done by going to the System Preferences themselves or by pressing Option-Command-F5 and then clicking the Preferences button in the panel that appears.
In the Accessibility preferences, click the Display section, and you should see a slider option for Cursor Size; here you can increase the cursor up to 4x its default width and height. While the maximum size is probably overkill in most situations, by boosting the size to between 0.5 and 1 tick marks (about 25 to 50 percent larger than normal), the cursor will be far more noticeable.
Even though the cursor will be larger, its accuracy will not be affected. The cursor should still work on a per-pixel basis and allow you to select items and move them as you would with a smaller cursor.