I have a pretty good eye for "proportion" of an Mbu puffer (size of pattern relative to eyes, etc.) and based on every internet photo I have ever seen, I can't say that I would put any of those fish at over 30". Of course, that is no proof of anything, just my experienced eye making a gut judgment.
Also, one thing I think is important in regards to tank size is the level of mental stimulation available to the fish. For example, even though my Mbu was cramped in his previous 4' tank, there were caves to investigate, other fish to watch, he was able to survey the living room and watch us, etc. To me, he seemed very "engaged" and was kept very mentally active. Now that we are in this apartment while the house gets done, and he is in a plastic stock tub that does have some rocks and a few fish to keep him company, he basically just sits around and does little. He eats fine, is healthy, etc., but I honestly feel like he is "bored" most of the time (my wife feels horrible for him and we cannot wait to get into the house and get him into the 300G) and he does seem less engaged when I am changing the water, rearranging the rocks. etc.
My point being that to fish which are inquisitive in nature and always like to be poofing around in the sand, in caves, or otherwise "doing something", like Mbu, I think that a giant aqaurium that just no decorations would be a lot worse in the long run than one that might be smaller-than-optimal, but was decorated or habitated in a way which better stimulated their mental needs.
Just my 2-cents.