I have lived with MBLs for nearly a decade. I absolutely love them, and have heard them in several rooms in my house, as well as in a number of other rooms, and , of course shows... and if stars align, I will upgrade to MBL x-tremes (hopefully a good number of years) before I die...
So from both passion and experience, I have to disagree with some of the responses above...The answer is, as with most complicated things in life, is that it depends.
First of all, you need a good balance of reflective and absorptive surfaces to make them work right... some folks complain of the bottom up balance of the MBL speakers and whine about overwhelming bass, while others cry about brightness... so getting this balance of reflective and absorptive materials in the room is key ...
In a perfect world, I would place the speakers on the long wall, if you can get away with it* (see below). If you do, your system will have a much larger soundstage, with instruments occupying distinct 3D spots on that soundstage, shooting dynamic waves of music from each of the spots on the soundstage, emulating the real thing
... This is exactly what the MBL team does at shows, and they always get mentioned as one of the top rooms anywhere and everywhere they present the system...
I would place the speakers at least 5 feet from the front wall (wall the speakers are closest to). Then play with positioning from the side walls. The farther you can get them away from the side walls, the better...
Toe the speakers in - notice the lamellas on the radialstrahler ... there is one that faces straight front on all three drivers. It also lines up with the mbl logo on the bass cabinet ....You want that lamella firing straight at your listening position.
The theory, and supposedly the MBL patent, is that this sound wave hits you first, then the rest of the waves generated by the other lamellas. This sounds correct to your brain just as an instrument, say a violin for example does when it radiates sound as it is being played. This set up is very important to getting the most from the radialstrahler.
Another cool thing to potentially get are some SMT wings.... acrylic ones are portable and will get the job done ....
They help focus the images by delaying the secondary reflection waves , so you primarily get direct sound from the speakers... there are some threads on them here ... also, if you get the SMT wings, you can get the speakers closer to the walls due to time delayed diffusion... (SMT says 0.6 -1.0 meter is good enough, but play around) ... SMT wings take an already incredible experience to the next levelll
But the problem with the long wall is that it's harder to control the bass.... Check out this :
Typical Long Wall vs. Short Wall Spikes and Nulls....
When the speakers are on the short wall, it's easier to control the bass peaks and nulls in your room... if you place a lot of absorption in the corners (intersections of any walls) in the first 1/3 of the room (as gurus like Art Noxon recommend) you will capture the bass waves "at the source", and the bass waves won't "wash over" the speakers and cloud things up... it's truly amazing how properly controlling the bass provides greater transparency and dynamics in the rest of the frequency range and transforms audio systems... kind of astounding so many audiophiles spend insane money on gear, while neglecting the objective reality of physics/ room acoustics)
So if you can get proper absorption of big, bad bass waves while having your system on the long wall, it's the ideal. But this is more complicated... Otherwise, go for the short wall, and you will have a lesser stage, but a more transparent and dynamic sound.
Out of curiosity, how big is your room? Where are the reflective and absorptive surfaces in your room? And what is your ancillary equipment, especially the amps? (4 high current amps is always better than 2, as it gives you near - horn- like dynamics and microdynamics).