WordPress.org

Download the plugin, upload to your Wordpress plugins directory and activate.

By adding the following to you theme, inside the body:
efontsize('85');
you can set the default body text size, a default of 100.1 will used if the value is omitted.

As seen on http://elfden.co.uk/ I used this style to position the links at the top right hand side: ul.efontsize{
position:absolute;
top:0;
right:0;
z-index:999;
}
ul.efontsize li{
display:inline;
margin-right:5px;
}