Default WordPress Font Size
Beginning in WordPress version 3.9 (with the update to TinyMCE version 4), the standard font size of the editor is measured in ‘pt’. Prior to WordPress version 3.9, the editor font size was measured in ‘px’.
Pixels are fixed-size units that are used in screen media (i.e. to be read on the computer screen). Pixel stands for “picture element” and as you know, one pixel is one little “square” on your screen.
Points are traditionally used in print media (anything that is to be printed on paper, etc.). One point is equal to 1/72 of an inch. Points are much like pixels, in that they are fixed-size units and cannot scale in size.
Kyle Schaeffer explains the differences very well in his article, CSS Font Size.
Set px as Default
Whatever the reason for TinyMCE making this adjustment, WP Edit Pro allows the option to switch the editor back to using ‘px’ as the default font size measurement.
Visit the plugin settings page, and click the Editor tab.