How to make a resizable goballoon

Through I set the Resizable property be true but O can’t resize it.

That’s true – the GoBalloon class (and GoComment, from which it inherits) is designed to have a fixed area around the Label, as specified by the TopLeftMargin and BottomRightMargin properties.
I suppose we could support resizing by having DoResize modify the value(s) of those two properties. I’ll add it to the list of possible future features.