The Font Size tool will always apply one font size larger than the one selected.
This means your WYSIWYG button for Font Size is configured incorrectly with respect to which CSS it applies when a font size is selected.
Steps to replicate
1. Create a solution
2. Enter the text "Medium"
3. Select the text "Medium"
4. Click the Font Size icon then click Medium
5. The selected text becomes the Large font size (not the medium font size) and remains selected
6. Click the Font Size icon, you can now see the font size with a "tick" next to it is Large
Looks like a dev forgot that arrays counts start at 0 not 1 :)
I also noticed a similar issue when attempting to change font type between Arial, Helvetica, and Helvetica Neue - toggling between the three does not change the font type.
Apologies for the pretty late reaction to the post. We've a new improvised kbase editor available now : https://support.freshdesk.com/support/solutions/articles/223670-introducing-our-new-knowledge-base-editor