This one might have slipped under the radar for some users but it’s now possible to quickly resize a plugin GUI without having to write any extra Csound code. All you have to do is add a combobox with mode set to “resize”:
combobox bounds(10, 180, 80, 20), mode("resize"), value(3)
This will create a resize combobox with 7 resize options, ranging from 50% to 200%. A recent update on this is that the MOUSE_X and MOUSE_Y channels will also be scaled so that any hit test code you have written will still work after resizing. As you can see from this video, the mouse X/Y coordinates remain the same even after resizing.