Lloyd,
This a code snippet for set the TextSymbols font size:
require([
"esri/symbols/TextSymbol", "esri/symbols/Font", ...
], function(TextSymbol, Font, ... ) {
var textSymbol = new TextSymbol( ... );
var font = new Font();
font.setSize("12pt");
font.setWeight(Font.WEIGHT_BOLD);
textSymbol.setFont(font);
...
});