The Signature and Avatar thread never explained this:
if your signature (.png) is spoiler'd, can it exceed than the ruler?
Nothing inside the spoiler is counted against the size limit.
So: I didn't get that answer. Can the picture inside the spoiler be more than 600*100?
Yes
Size limits don't apply to anything you put inside the spoiler.
However, you should also be mindful and make sure your size image inside the spoiler isn't excessive (ie: you don't make everyone download a giant 8 MB image, true story).