Obviously not large enough if you can’t tell what it is selling!
I suspect Google are just trialling a new format and it wouldn’t surprise me if they reverted back to a smaller size within a week or so. Since the XenForo update I have had the ads set to ‘responsive’, which means they scale to the device, rotation and available space. I’ve not seen them this deep before, but its not much of an issue as you oly see them at all on first visit to a thread, if just jumping into read new posts you miss the header unless its a new page, and most folk don’t get as far as the footer. I’ve so far resisted putting mid-page ads like most forums use even though they would likely double (or more) the revenue, so in the grand scheme of things pfm still has a very low advertising footprint.