Font size inconsistencies among different browsers

Lately I've been working on making my sites more cross-browser compatible. I find that I can generally get the main elements of the site to be almost identical, however when it comes to fonts, things come out rather inconsistently. The biggest difference I see mainly comes out on an iPhone, specifically on header elements, however other browsers share similar problems. Does anyone know of any method to make font-sizing more consistent among browsers? Thanks to all who help.