On a Mac, go to your System Preferences and select "Accessibility" under the System panel (on older operating systems, this might be listed as "Universal Access").
Then, just drag the cursor size slider until your cursor is the size you want.
You can also click the button at the bottom of the panel to show Accessability status in the menu bar at the top of your screen. This way, you can switch between a large and regular-sized mouse easily without having to open your System Preferences each time.
It's been at least 10 years since I've used a PC, but this should work if you're wanting the same effect on a PC:
1. Click on the Start button, select Settings, then Control Panel.
2. Double-click the Mouse icon, then click the Pointers tab.
3. Within this Tab, you should see a drop-down box labelled "Scheme". Look for an item labelled "Windows Standard (large) (system scheme)", or if you want really large cursors, select "Windows Standard (extra large) (system scheme)"