Add the w3-small, w3-large, w3-xlarge or w3-xxlarge to change the size of the navigation (will change the font-size of each link).
Default:
w3-large:
w3-xlarge:
w3-xxlarge:
If you want more space between each link, instead of increasing the font-size, you could add a w3-padding-classes to each link:
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