You can go to Dashboard > Theme Settings > Typography > Advanced > there create a new rule by clicking on + Add Rule > then in that select Main navigation links > then you can define there the Font Size as well as Text Transform (cases) properties too which will work out for you: https://ibb.co/bWv1Mp6
You can try out the following Custom Selector in this regard on your Site under the Dashboard > Theme Settings > Typography > Advanced > + Add Rule > in that you need to select Custom Selector > then you need to paste the below-given CSS class:
.wd-tabs-header .wd-nav-tabs-wrapper ul li a span
After that, you can set the Font adjustments accordingly. See this image for reference: https://ibb.co/5sQpmT3