Well I'm not Terri, but the 28qts she uses look to be the same ones I use. The model number is 1856. The dimensions are: 22 ¾" x 16 ½" x 5 ½".
However, Sterilite has a new model set out, 1956 would be the new 28qt with dimensions of 23" x 16 ¾" x 5 7/8".
For me, they're too small for a typical adult corn snake. It just seems extremely cramped to me. Plus, if they were to have a monstrous poo, there would be no avoiding it. There just isn't enough room in there for a single hide, water bowl, and an adult snake. I'm sure some people do it, I don't.
I use the 28qts for juveniles and small adults. It seems to fit them comfortably. However, if you're not using them in a rack, the lids are
extremely flimsy and when coupled with a smaller statured snake, the risk for escapes are a lot higher. Trust me there.
Picture of my 2003 Hypo Lavender in a 28qt (Yes, I know she's small. Its seemingly beyond my control as she eats like a horse, just grows slowly)
I don't think there is a "set age" to move them up. Move them up when they're looking cramped in their current container.
I typically move hatchlings out of their shoeboxes when they're feeding on fuzzies, then they go into 12qt Sterilites. When they're feeding on weaners or small adults they go into a 28qt. When they're handling adult or large adult mice, they go into my 41qts or the 96qts (I believe it is). I can't find the 96qts anymore. They have the same footprint as the 41's, but are like 18" tall, rather than 6" tall.
You can see the 96's on the left of this picture, the 41's are holding adults in brumation atm:
Hope that answers your questions. =)