Signature sizing issue

I am attempting to update my signature, as per the forum rules (and the Signature Settings page), the maximum dimensions are:

Quote

Signature Size:
Signature images should be no more than 600 pixels x 140 pixels in size. If you choose to include text underneath an image please limit it to three lines, including spacing, on default text size. However, if you wish to just have text (and no image) you may have up to 6 lines including spacing on default text size. The default text size is 2

However, the image automatically resizes to 300 x 80 and will not allow modification:

Which restriction is the actual limit and/or what's going on here?

Using Firefox.

EDIT: A workaround is to manually use [.img] tags, and that seems to be fine. But still, the dialog should be fixed.