when you say built on wordpress do you mean you host it yourself and it uses wordpress or its hosted by word press You have access to the CSS style sheet in the back end by going to appearance - editor This will automatically open up the style sheet of your template and then you can edit the css The code for font size is font-size:22px; change the 22px to what you want BUT PLEASE before changing the style sheet BACK IT UP make a copy
You can also download and install the plugin below and it will let you change the font sizes without touching any code or style sheets. Just another option. https://wordpress.org/plugins/tinymce-advanced/
I think you'll find the answer to this question here - http://www.templatemonster.com/blog/ . I've learnt so many useful hacks from that blog. Btw, I remember for sure that I've read a great info about fonts there!
In your dashboard, Goto Appearance and then click theme options, there you can find the font size of your website. You may change it over there, as per your pixels.
Vegee is right. You don't change font size in "Wordpress" but in your "Wordpress theme", whatever that may be. All themes have options and most of them are giving you an option to change the font size. But it often doesn't work, at least I always somehow end up editing the theme's CSS files to get the font size I really want.
Honestly, I prefer using some free website builders instead of WordPress. Here is some info on the best website builders available for free - http://www.beautifullife.info/web-design/15-best-free-website-builders/ . Personally, I prefer using Wix templates, so I can just swap in my content to get started really quickly.