If you posted yout Gimp file it would help, but is sounds like the arrow layer needs to be resized. Either layer->resize layer, or you could use the scale tool in the toolbox
https://www.youtube.com/watch?v=sFJAE-oriQk
The unified transform tool is most versatile
https://www.youtube.com/watch?v=sFJAE-oriQk
The unified transform tool is most versatile