If you have something on the screen that you would like to get a closer look at, you can zoom your screen in by simply holding the 'Control' key down and rolling the scroll wheel on your mouse. This should zoom into the screen and resize everything to be larger.

As you zoom, just move your mouse in the direction of the area you wish to view.

If your Mac doesn't zoom as expected, then go to Apple menu → System Preferences and select Keyboard & Mouse and then the Mouse tab. Be certain the box labeled, Zoom using scroll wheel while holding... is checked.