get highest fontsize in textrange

I use word automation in my Vb6 app.
I use a textbox shape

VB Code:

dim TBox as shaperange
set tbox = Appword.selection.shaperange
'-if one textbox shape is selected
if tbox.count = 1 then
 '-I want to get the highest fontsize, but more fonts can be used
 tkFontSize = tbox.textframe.textrange.font.size
end if

Now i only read the current fontsize, but if more fontsizes are used i want to know the highest fontsize.
I know i can loop through all the characters of the Textbox shape but is ther a faster and more efficient way? Is there a collection of the fontsizes of the textrange of the textbox?