Open a sitemap and navigate to the toolbar on the left, above the work area. On the right side of the toolbar, you will see the ‘zoom’ button.
Click the + button to zoom in. You can zoom in 150% at most.
Click the - button to zoom out. You can zoom out 50% at most.
Optionally, you can use the following hotkeys to zoom in/out:
Press CMD/Ctrl + to zoom in or CMD/Ctrl - to zoom out.
Read more about keyboard shortcuts here.