Label does have a fontSize property (and fontFamily etc). see: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/spark/components/Label. html
Also, none of the spark text components have a backgroundColor or backgroundAlpha style - instead you should create a custom skin.
But why don't I have it then
I have Flex SDK 188.8.131.5286.
Can it be accessed through AS code directly, or maybe, I have to refer to a styleManager or something else?
yep. its a style, so use eg myLabel.setStyle("fontSize", 12);
Oh, that's it. Another foolish question :]