Generally it doesn't matter what size/ratio your displacement is aslong as it has 4 sides. I would also not recommend making them huge either, as a matter of principle and ease of use.
Its not a big deal if you make them from 1 brush also. even if you have destroyed a displacement you can easily make that same face a displacement again. There is a [DD] button (displacement drawing). when you click on it all the faces that get removed when making a displacement still show up and you can make a displacement of them again.
I personaly make as much faces of a brush a displacement without overdoing it. It often keeps my map structure easier as i allways vertex edit my brushes to have them perfectly aligned. This means that i dont get any weird faces when i press the [DD] button anyway.
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