Ugh. This always gets my goat as well. I realize that certain sizes are "popular" and tend to sell out quicker -- sooo . . . why don't they simply order and stock more of that specific size?!
I recall one time that I was in AK, I went into a gift shop and loved a t-shirt designed with a bunch of jungle animals. Of course, my size was sold out in the gift shop.
The CM nicely called over to AK Lodge, and they actually had one left in their gift shop. I headed over there via Disney bus, just to get the t-shirt! Made the best of the diversion in my plans for that day, and had lunch there in The Mara -- (IMO, the best QS on Disney property), and made sure I got some zebra domes for dessert, too!