but gimp doesn't show the complete text, because the alpha layer is smaller than the text size. How to scale the united states flag alpha layer without scaling the image itself ? thanks
As far as I can see, it's not the alpha channel that is the problem.
The real problem is simply that you have created a text layer which is larger than your current canvas size and GIMP will only show that part which is visible within the canvas.
Just do Image > Fit Canvas to Layers to automatically re-size the canvas large enough to include your text layer, or Image > Canvas Size.. to set the overall image size to specified dimensions.