I did took three screenshots of my use of the "+" and "-" icons.
- a "before" of the textbox
- an "after" of the textbox when clicking on the "+" icon
- an "after" of the textbox when clicking on the "-" icon (when at the original textbox size)
I then cropped each screenshot so that it was automatically sized ("Image > "Crop to content" option in Gimp) to only be the size of the textbox.
The height of the original textbox was 636 pixels. The height of the textbox after clicking on the "+" icon was 646. The height of the textbox after clicking on the "-" icon (when at the original textbox size) was 626.
Based on that, the "+" and "-" icons increase or decrease the height of the textbox by only 10 pixels.
For these two icons to be useful (especially since the size does not seem to be remembered), the incremental difference to be at least 50 pixels. My opinion would be 100 pixels but I understand that could be too much of a difference for smaller screen sizes.
-Doug