Magento 2 logo change & logo size
Your current store theme determines the size and location of the logo by default. The default Magento 2 logo is saved as an SVG file. The desired logo can be saved in any of the file types: GIF, PNG, JPG, or JPEG and uploaded from the admin panel of your website.
To find the default Magento logo and change it for a new one, navigate to:
The Admin Panel> Content> Configuration> Select Your Current Theme> Header;
!For Magento 2.x.x - Magento 2.1.x use this path: the Admin Panel> Stores> Settings> Configuration> General> Design> Header!
It is always better to keep font and logo small. It's easier to read on a small device. The fonts are also not as legible on a small screen. Fonts also tend to be more readable when they are smaller. Visit this https://masterbundles.com/girly-fonts/ site for premium girly fonts. I am not sure if this would work for all mobile sites, but I think it is better to test it out on a few different sites and see what works best for each site.