This isn't only about size and material Nakeen, it also about the power the portal has. Should making a single mundane tree be slower than making a wooden artifact that is smaller than the tree?
As we diverge on an important point, which to me seems to be the relation between Power and Time, let me explain my view.
To me Power doesn't equate to more Cast/Creation Time, and doesn't need to. For many reasons, with some more justifiable than others.
To you, I assume that Power should equate to more Cast/Creation Time, for what I presume is mainly balancing purposes.
So we know that Island Creation take some time to be created, due to being Matter Creation.
But the Portal Creation is more akin to a Magic Spell.
And unlike the Matter Creation, we don't actually know how or if Magic is affected by Time.
To one, maybe Magic will require a Cast Time proportionally or exponentially greater with the overall "Power" of the spell.
To another, maybe Magic will be quasi-instantaneous, with Cast Time having a minor or logarithmic increase depending on "Power".
A small image to picture how I see these two point of view and their basic ways of balancing/controlling abuse.
So someone asks for a big stick to hit something.
In the first case, the big stick will be placed on a conveyor belt, with the someone waiting at the end. In the meantime, everyone else can just push the big stick out of the conveyor belt, requiring for the someone to ask another big stick.
Here, it is safe to assume the someone will never have his stick. And those who will have theirs will be unstoppable.
In the second case, the big stick will be handed directly. To stop the someone from hitting the something, another person will have to use another big stick, or a bigger one, to either hit the someone or the stick itself.
Here, it will lead to a big stick battle, ending when one will be exhausted or have broken all its sticks.
That's how I see the matter.