Well, you could set the foreground color of the group box to transparent so that the title text becomes invisible. Then create a label with the font size and color etc. you need. You cannot simply drag and move the label into the group header area because the editor will snap the label into the group, but if you place the label directly in front of the group then change the pixel position (Left, Top), then you can force the label to overlap the group box header and achieve what you want (or close to it).
I don't know a way to increase the title height of the group box though so your selected font size should fit into the default height of the group header...