It is not currently possible to manually set the font size for a Text Datablock.
You can select the Font size by selecting H1, H2, H3 from the Datablock Editor on the right-hand side of the Designer.
The font size on Text Datablocks also automatically resizes based on the size of the Datablock. To change the font size, it's recommended that you resize the Datablock or change the amount of text on the Datablock.