Unlike most other GUI controls which express the font size as a percentage of the "standard" size, the XTEXT control, being more like a "real" text editor or word processor, prefers to use printer point size units (e.g., 12 for 12 point type.) You can also specify the font size in twips, in which case set TXC.FONTSIZE to negative the desired number of twips in height.
If you specify a non-blank font (in TXC.FONTFACE), you must also specify a non-zero size (or else you will get microscopic text). Note that regardless of the specified font size, the user can increase or decrease the display magnification using the Ctrl+Plus and Ctrl+Minus keys, or using the zoom dropdown on the toolbar.