I'm not comfortable with posting on that thread since it's pretty old, so in response to your latest post on that thread (the one with all the images):
Going by the colored bars you posted, the minimum width for the bodies of forum posts (so not including the user info)--and, therefore, the signatures in forum posts--seems to be between 500 and 600 pixels, and the maximum seems to be between 700 and 800 pixels. I say this because when I zoom out, the post widths don't increase past a certain size, and when I zoom in, the post widths don't shrink past a certain size, but they do increase or decrease depending on the width of the screen. (This is using desktop mode.)
For example, mental racat's signature clearly changes on my screen when I zoom in or out. If I'm zoomed in to the max, then the pictures squish the text between them so much that the word "anyway" is below the pictures and misaligned, and I don't see the text about guessing the dude in their avatar. But as I zoom out, the width of the text area becomes wider and I can see more text, and eventually the text about guessing the dude in their avatar is comfortably between the pictures. But once I hit the maximum post width, the signature no longer changes, and the text about guessing the dude in their avatar remains at the shoulders of the dudes in their signature, even when I'm zoomed out so much that the letters are all just dots xD
Going back to DaDwarf's last post in the thread she linked:
Image info tells me this for your big image (with all the buttons but not yet cut up):
Dimensions: 800px × 386px (scaled to 739px × 356px)
But this is when I'm totally zoomed out. I get something different when I'm totally zoomed in:
Dimensions: 800px × 386px (scaled to 531px × 256px)
Similarly, for the small strip image you posted ("Please note im on mobile sometimes!"):
Dimensions: 786px × 32px (scaled to 739px × 30px)
^ for when I'm totally zoomed out, and
Dimensions: 786px × 32px (scaled to 531px × 22px)
^ for when I'm totally zoomed in.
It should be noted that at different zoom levels past the maximum and minimum post widths, the width seems to vary by 1 or 2 pixels. e.g. I'm already zoomed in far enough so that the post width does not decrease any more, but when I zoom in a few more times, I get this for the strip image:
Dimensions: 786px × 32px (scaled to 530px × 22px)
So I guess it's reasonable to say that on the desktop layout, the minimum width is 530 pixels while the maximum width is 740 pixels. (and it makes more sense that the limits would be nice multiples of 10 haha)