I understand fully what you say, but the question was asked “ how can we crop in designer, right? “ So I was thinking how Illustrator handles it, in Illustrator it really does not matter at the beginning what you set your artboard too, you can always click and drag to a new size at any time, plus you will be able to see the new size in the context menu at the top!
with Designer which is similar to illustrator, this is a little more difficult or at the very least not as intuitive, and I do know that this is only because it is new, I am sure that as time passes this product will mature like any other.....