See the Font API. You'll find the constructor
Font(String name, int style, int size)
-Creates a new Font from the specified name, style and point size.
So you could do something like
String family = textFlow.getFont().getFamily();
int style = textFlow.getFont().getStyle();
int theNewFontSize = 30;
textFlow.setFont(new Font(family, style, theNewFontSize));