I strongly recommend that you don't use "Style" in Mendix, it makes your app difficult to manage on the front-end.
In any case, it is better to use a CSS Class and implement it correctly. Here you will find documentation about the front-end part of Mendix: https://docs.mendix.com/howto/front-end/
You could try using a Class from Mendix Atlas Design Systems on your reference selector eg use "h1" as Class and your font size will increase. (https://atlasdesignsystem.mendixcloud.com/p/texts)
However, the problem is not necessarily in the size of your text but also the size of your Reference Selector. Look, after applying the class, my text does want to get bigger, but it is not possible in the Ref selector: