How wide can an image be before it's resized?

Question

When adding images to questions and answers, what is the maximum width it can be before it gets resized?

Answer 1

640px is the width of the images. So that's the size they get scaled down to.

As from this CSS rule:

.post-text img {
  max-width : 640px;
}

Please note that this also illustrates that images are not scaled down server-side. They are always original size, just displayed smaller.

Of course, I myself have that overwritten, see Images can be pushed outside the boundaries of a post by using nested lists