I do this caveman-simple style;

I go to File>New Document.
I make sure "Constrain Proportions" is unchecked.
In the "Document Resolution" I type in 1600 x 900. (thus, an instant 16:9 ratio)
I then click again on "Constrain Proportions", now making sure it is turned on.
I then change the width of the "Document Resolution" to 1024.

That's it.

*A width of 1024 is the width I build a scene at, because that way I can see the whole scene at once. (at my monitor resolution of 1280x960) This is best for staying on top of your composition.

*When I'm done building the scene, I render out at a wider with (i.e.; 2880. - the height will take care of itself as long as "Constrain Proportions" is still checked )

AS

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