If you wanted a long narrow piece of land changing at the moment you would have to change it one square at a time. It would be good to be able to change the length and width of the brush independently.
I've done a mock up of how I see it working.

I think a spinner might work better, but otherwise I like this idea.
Or a selection area like Word uses for tables.

Or making it a two step interaction: first select the land area, then move it.
I'm not a huge fan because I like it being square.
Perhaps just change the input box to take two values instead of one so the current widget (what's that kind of selection thing called?) can stay for those who like it, and change it to something more informational when the brush is not square.
What about adding a button that allows the user to select the shape of the brush, then displaying the setting necessary for the shape of the bush.
As explained better by me mock up below.

This way we keep the current widget but add the option should the user want.
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With the surface paint merge we can actually change the logic for it to do rectangles if so required.
I don't think you would really need a rectangle selection if you instead had a "flatten" tool, which you could drag around, smoothed or non-smoothed.
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What about adding a button that allows the user to select the shape of the brush, then displaying the setting necessary for the shape of the bush.
As explained better by me mock up below.
This way we keep the current widget but add the option should the user want.