Gimp question: Resizing part of an image

[Joel Hammer]

I don't know where to start looking this up in the gimp manual.
Is there a way to select and resize part of an image?
The idea here is to insert a ruler into an image which could be scaled
to match the image. I would take the picture using some nasty, dirty,
foul looking ruler, made from brass which nobody has bothered to polish
for a while, likely covered with fresh blood and bile, next to the object
being photographed. Then, edit the image, replacing the real ruler with
a fancy one (an immaculate white on a pure black background with our
hospital logo, or some such). The only question is scaling the new ruler.

Joel