I don't personally find it too big to read, but I will go out on a limb and propose that if the main goal of your site is to provide text content to readers, don't set a font size at all.
Unfortunately I don't know of any way to get a mobile browser to display at a normal size. It seems that both Android and iPhone are configured to use microscopic text as default to guarantee a desktop-like layout.
Their default text size isn't that small, it's just that they render pages to a larger surface than then display by default, in order to avoid breaking old sites. This can be avoided by adding the appropriate meta tag to the site, but that has to be done by the developer.
Unfortunately I don't know of any way to get a mobile browser to display at a normal size. It seems that both Android and iPhone are configured to use microscopic text as default to guarantee a desktop-like layout.