@kmevanish,

Well, you can change the font size of the main menu by putting the following custom CSS code in customizer
section of your dashboard.

.main-navigation ul li a{
font-size: 16px; 
}

Note: Change the above numeric value as per your desire.

Hope this helps !!

@kmevanish,

Well, you can change the height the main menu by putting the following custom CSS code in customizer
section of your dashboard.

@media only screen and (max-width: 1023px){
	.main-navigation ul li a {
    padding: 17px 13px;
}
}

.main-navigation ul li a {
    padding: 17px 25px;  
}

You can change the value of padding: ie value 17px to your adjustment.
Hope this will solve your issue.
Let me know how it goes.

Regards!!!