"The default is to just have the font size slider. If you go into System Preferences, Accessibility, though, and turn on 'Reduce transparency' the preferences in Messages change to allow you to pick a particular font. (Close Messages before making the change in System Preferences.)
Yes, the actual Preferences dialogue in Messages changes depending on what your transparency setting is." -Orc4hire
I TOTALLY see this as representative of just how intuitive* Apple REALLY is... LOL!
Or, perhaps, as "Think Different."
Massive Thanks for knowing this.
SERIOUSLY.
As soon as I returned the "reduce transparency" check to its box, my font choice and size were returned to what they had been. Interestingly, other font size choices elsewhere on my computer returned to where I had set them for various applications and uses. I think that I had un-checked the box when messing about with various system pref's after reading something that another forum user had complained about and I was trying to sort a fix. I wondered why my Messages font and size wonked out... had absolutely NO idea that transparency was tied to font size.
crazy.
Those sly Apple people.
What WILL they think of next?
snicker.
p.s.
however DID you sort that answer?
ESP?
CHEERS to you!
*not even a little bit thinly veiled sarcasm. this was BLATANT sarcasm. and only the bit next to the asterisk. the rest of the post was not sarcastic. I'm only making this footnote so that no very sensitive persons become nervous about just what I meant by that.