yes u can change the size of the duct label. Ok type ST (text style) on the command line. make some text styles (example) new text style, then pick the font you use, then put the height, apply i.e CRS 7.5 (height) 7 1/2". Then go to Format (pull down) click Style manager expand Documentation Objects click Label Curve Styles right click select new New. Now Double click on the one u just made and customize it. make sure on the Label Style Annotation tab to pick the text style u made i.e. CRS 7.5. If u need to see attachment let me know and I will make some.
Thanks