The method in @Billy Kerr's answer is working, but I'd like to share an alternative method I have been shown in the meantime. The following solution has been posted by @Wormnest here.
Solution (literally cited from the link above):
- Assuming the rectangle select tool is selected:
- Check the box that says "Fixed", the default is usually Aspect Ratio; change it to "Size".
- In the edit control directly below it write down the dimensions, e.g. 100x150 (with an x between them). As an aside, it would be nice if GIMP could prefill it with the current selection size.
- If you want to save these dimensions you should save it as a preset. If you only need it while GIMP stays open then you can do:
- Switch to another image.
- Click where you want with the rectangle tool. It should have a fixed size selection.
I can confirm that this solution works, although it is very worrying that the selection in the first place does not change when you enter new dimensions in the "Size" field. But when you create a new selection, it has the dimensions expected.
The method is really fast once you have set the correct rectangle selection tool options: In each image, you just need to click and move the selection to the correct location. I hardly can imagine a faster method (well, when defining the initial selection, the "Size" field could be pre-filled with the values of the current selection :-)).