If you are using the single-window mode, the Tool options dialog is part of your window. If it is not there or is detached and floating around, see
here.
However, if you want a dimension that you know in advance, you can set the tool to "Fixed: size" and enter a size as "500x1000" for instance. And if you use that size often, you can save that "preset" with a name, and reinstate it at will (see the row of icons at the bottom of the Tool options dialog).
Attachment:
RSTO.png [ 45.25 KiB | Viewed 88 times ]