Hello Rose,
I love your excitement! Great feeling isn't it?!! You are going to enjoy that young thayeri pair headed your way and from your question it appears as though you already know you won't be satisfied with just two! 
I've never particularly been a big fan of keeping snakes in plastic tubs, but as many generally do I ended up that way. I primarily use a rack system... shelves with the shallow plastic tubs. Now that I am here, I clearly see the advantages and appreciate them but I am doing things on a fairly large scale. The primary advantages are ease of cleaning, and if the tub is ventilated properly the water bowl maintains a more appropriate humidity level. A clean, dry but somewhat humid environment is a plus. It really depends upon what you plan to do with your collection, how big it gets, how much space you have and how much free time you have on your hands to manage it.
If I had the expanse of space required and the massive amounts of free time involved in cleaning and maintenance, I would prefer all the animals to have large display type enclosures for a better life but it wouldn't be practical for me. I do still maintain 5 of these for a very lucky select few. Nice to look at, you can give the critters lots of things in cool and warm areas to hide in, climb on and investigate and they love it. I would recommend you stay with something like this if you only plan on a small thayeri collection, they will have themselves on display most of the time and you will enjoy them more.
Most references suggest a minimum enclosure size which will allow the animal to stretch out fully around the perimeter and cover no more than 2/3 to 3/4 of that perimeter length. Of course, they will appreciate much more room if you can give it to them and you will see a more natural behavior which is interesting. My display tanks are fairly large and my plastic tubs are big enough to allow them to stretch out around the perimeter and cover no more than 1/2 of it. And being snakes, they don't seem to mind too much that the tubs are shallow but you are absolutely right with your thinking... I wish I could give the tub animals more room and things to climb around on.
Mike