There's no option for that. Text layers are the size of the comp even though the text can extend beyond the boundaries. The only solution that I know of is to add a slider control to the text layer then add this expression to the anchor point:
[0, -.35*effect("Slider Control")("Slider")]
The .35 factor is about where most have the center of their height. You then just adjust the slider to the font height. You can tweak the value if it doesn't fit your particular font.
Unfortunately there's no way to directly access the font size with an expression
You are not layout people or graphic artist so this may have passed you by but there is a thing called baseline shift and i think this came up a very long time ago as well.
here is a little snapz pro movie easier me to show then to write it out.
One thing I had totally forgotten, is an expression using sampleImage(). There's one such project floating on AEnhancers that detects the text bounding box. If you expand a little on that, you could modify it to use half the text height.
That is the solution.
I am a "a layout person and a graphic designer" actually.
And yes baseline shift works perfectly.
So if you center-style the text, slde the baseline shift, and BAM, you are centered.
Its funny to me that there isnt a anchor point orientation selector like there is in Illustrator or InDesign.
I spent over an hour poking around looking for it.
I guess it doesnt exist in AE.
I really should.