AP calls for life-size, so Mr. Rewrite assumed – incorrectly, it turned out – that that would mean correcting the reference to medium-size trucks.
The AP Stylebook offers no direction on the subject but does send users to the Webster's New World College Dictionary, Fourth Edition, for guidance on matters it doesn't cover. The dictionary says to use sized in compounds referring to things with specified sizes, listing medium-sized and undersized as examples. It lists life-size next to an entry on -size.
Based on this, Mr. Rewrite is going to apply -size to uses that don't specify a size, such as the concept of value in the picture. For this, he recommends "value-size cooler pack." He forgot to add a hyphen to the photo, by the way.
To reward himself for cracking this usage conundrum, Mr. Rewrite is going to supersize his dinner tonight. Or something like that.