There are no standards for this sort of thing, estes. The people buying the art are likely to have as little idea about the "right" way to do any of this as you. The quality of the art is what is most important.
When in doubt, you can always say 16 inches wide by bla.
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It doesn't really matter to be honest, it's the same size of canvas either way. I'd probably go with width x height personally, just cause that's how a computer resolution is described, but when they look at the work they'll know how it's orientated and so will know what the smaller size is
Oh, and there's no such thing as a dumb question. Asking questions is how we learn more, which is always a smart thing to do