Let me preface this by saying... This is just my opinion...
Here's the thing about WD artwork... The recent spike in demand correlated with the television shows popularity. It had nothing to do with how brilliant and well executed the comic book was. This leads to me to believe a lot of the people paying crazy money for the artwork are the speculator comic collector type, not the art collectors whom are in this hobby for the long haul type... Should the TV show ever go off a cliff the current pricing will as well. Anyone remember the effect the first two seasons of Heroes had on Tim Sale artwork? I do. His stuff sky rocketed. And as the show's popularity wained, his artwork came back down to normal levels with the exception of Long Halloween and Dark Victory, Superman All Season, and DD Yellow and Spider-Man Blue. Once WD the TV show is over I suspect the prices for a lot of WD art will dwindle as well... Especially if a new zombie property appears that steals everyone's hearts.
That being said, the book is strong enough to stand the test of time, and once the show is over, it wouldnt suprise me if slowly but surely the value raises in price in a normal non inflated way... But that too is just speculation. Only time will tell.
As for the size of the artwork being an issue. it probably is to comic art collectors... But to the whole speculator comic book crowd whom I think is the majority of people causing the demand, Im sure the size of the art board isn't an issue.