For example, the original size is 800x800. I want to change it into 400x400, preserving only the upper left corner and discarding the data outside the new region. In mspaint I can do this via File->Properties, but I can't find a similar function within gimp.
From GIMP documentation 4.5. Crop An Image:
Use menu Tools > Transform Tools > Crop in the image window and enter in the dialog :
- Position : 0 , 0
- Size : 400 , 400
Double-click inside the selection rectangle or press Enter to complete cropping.