Hello @james-salisbury,

You can change the font of the site title and site tagline by going to Theme options > Font Family Options > Site Title and Site Tagline .

Similarly to change the font size you can use below given CSS.

For this go to Admin Panel > Appearance > Customize > Additional CSS and paste below given CSS.

.site-title {
    font-size: 22px;
}

.site-description {
    font-size: 12px;    
}

You can adjust the font size according to your requirement .

Also, the feature to add the phone number in the header is not available in the theme . As much as we would love to help you this is beyond the support we offer for our product .

Our suggestion to you is to hire a professional developer for your customization, so that your theme will no be effected in any way.

To hire a developer please follow the link below
http://themepalace.com/hire-a-customizer/

We hope it helps,

Best Regards.

Have a good day .