If you go by what I've seen/read - roughly a bottlecap per acre. And, you assume that bottlecap is roughly a cubic inch in size. And that the trash patch is the size of Texas, which is roughly 172 million acres....
All the plastic in the "Great trash patch" would fit in a little over 300 dump trucks. That's a pretty small pile of trash, in something the size of Texas.
I remember several years ago a discussion about just how much land would be required for trash dumping and it turned out it wasn’t really very much in the overall scheme of things.