I propose to move that criterion to Final milestone instead, and use this wording:
"The release-blocking images must meet current size requirements, as documented at [Releases/28/ReleaseBlocking][1]." (the release number will of course change in time)
I chose [1] over [2], because [1] contains all important release-blocking images (distinguished to individual .iso or .img level), while [2] only contains spins and nothing else. In [1], we will add a new column labeled "target size" and fill it out. It is enough to fill it out just for images marked as release blocking. The nice thing is that in that table you can easily see who is responsible for that image, and so it's clear who's responsible for keeping the target size up-to-date (and also whom to ping if it isn't).
Note that I haven't proposed any particular sizes for any particular images. I believe that should be all the responsibility and decision of the owning group (or perhaps FESCo, for some generic images), not QA.