Font size extremely small on mobile devices

Depending on the version of IOS you're using it may actually be an issue, with that version of the browser.

Just declare the base font size:

body {
 ~mybase font size
}

Have you added the viewport meta tag to your document?

If not, add this to your head section:

<meta name="viewport" content="width=device-width, initial-scale=1">

A mobile browser will naturally attempt to zoom out to display your entire web page like a desktop browser. The viewport meta tag scales your page to the device width.

More information:

• A tale of two viewports ~ QuirksMode.org
• Using the viewport meta tag to control layout on mobile browsers ~ MDN