I suspect that you guys are probably noticing it so much because these are very much “small sig” forums. I mean take my sig for instance; it’s so small you can hardly see it. It’s very interesting of course, and without a magnifying glass or a microscope you’re really missing out…
In all seriousness though, there don’t seem to be any written guidelines for image sizes in sigs; I noticed a text limit guideline, but I couldn’t tell you how many pixels 255 characters adds up to off the top of my head, or if that’s even meant to be applicable to images. Image sizes should probably get their own mention somewhere on the FAQ / Profile pages if they can potentially be an issue…
Well the issue is that one post with a sig like that is like 3 posts of other ppl with small sigs so it makes you have to scroll too much eventually, it makes the forums harder and more unpleasant to read IMO.