First of all we need find  one theme file in folder tiny_mce \jscripts\themes\advanced\editor_template.js in Windows or  /jscripts/themes/advanced/editor_template.js in Linux But keep in mind that you maybe have another path to this file themes/advanced/editor_template.js .
Open it  find and replaice next things :
sizes : [8, 10, 12, 14, 18, 24, 36]
to
sizes:[10, 12, 14, 18, 24, 30, 36]
next
theme_advanced_font_sizes : “1,2,3,4,5,6,7″,
to
theme_advanced_font_sizes:”10px=1,12px=2,14px=3,18px=4,24px=5,30px=6,36px=7″
next
m.font_size_style_values=”8pt,10pt,12pt,14pt,18pt,24pt,36pt”}
to
m.font_size_style_values=”10px,12px,14px,18px,24px,30px,36px”}
next
if(p==q&&q>=1&&q<=7)
to
if(q>=1&&q<=7)
next
(“+l.sizes[q-1]+”pt)
to
(“+l.sizes[q-1]+”px)
next
(l.sizes[q-1]+”pt”)
to
(l.sizes[q-1]+”px”)
next
(l.sizes[q-1]+”pt”)
to
(l.sizes[q-1]+”px”)

thats all :)

Just save it now and reload your page with your changed TinyMCE. You have px instead pt in your TinyMCE now  ;)

tiny_mce\themes\advanced\editor_template.js