Change the font size of the control text

I would like to change the font size of a text note to smaller than 'h6'.

I don't know if I should add something in CSS or if I can do it purely with html
thank you. I wait for the answer!

I would like to change the font size of a text note to smaller than 'h1'.

I don't know if I should add something in CSS or if I can do it purely with html
thank you. I wait for the answer!

The note that says: Este campo debe contener un número telefónico sin 0 ni 15. Ej: 112653009

Dear @belen_costantin,
It might be easier to just use HTML control, where you have complete control over appearance of the text.