Obviously, you need enough room. If you're only talking about four to eight nodes, you might well be able to build it in your office (presuming that your office is no smaller than mine). In fact, my current home beowulf is ``in my office'' - three nodes under desks in various rooms of the house (one of them mine) and a small stack of nodes next to the desk. I could easily stack up six to eight EN7653 mini-mid towers next to my desk. I probably couldn't stack up sixteen for a variety of reasons, and would have a hard time with even eight full size towers.
If you are planning to stack more than two units high, you will likely want shelves so that you don't have to unbuild your whole stack to get to a single node in the middle for service. My mini-mid towers are roughly 8x18x18 (inches), so I need at least two feet of space from a wall (to allow for cords and stuff) 16 inches wide and around 36 high to build a four node stack. With open shelves, I'd probably add two inches horizontally and four to six inches vertically to make it easy to get nodes in and out and allow a bit of air to circulate around the boxes.
A BIG shelved beowulf is most conveniently built well away from any wall, so that you can walk right around it. That way you can easily get to the back and do the cabling, can slide units in or out for service, and can keep air flowing through the whole thing to keep it cool. The same is generally true for rackmount beowulfs - even though the racks are often on wheels or casters, it's preferrable not to have to move them to get to either the front or the back.
Make sure that the floor of your space is in fact strong enough to support your beowulf. A loaded PC case might weigh anything from 10 pounds to 40 pounds (power supplies, especially, can be very heavy) and the shelving (if any) adds to the weight. Stack up enough of them on a few square feet of an upstairs room of a woodframe house and you can make the house quite unhappy. Remember, it's so embarrassing11.3 when all that hardware (and you) plunge down on your pets and children in the living room in the event that you fundamentally overload the structure - I personally just hate it when that happens.
Even in an office-type building, if you crank three fully loaded full-size racks into a small office you can have a similar experience. Estimates published on the beowulf list for the weight of a fully loaded rack (I asked!) can range anywhere from 700 to perhaps 1500 pounds and even if nobody gets hurt when it crashes through the floor it would be an expensive mistake. In a lot of cases, organizations with a rackmount beowulf will also have a big UPS, which can also weigh in close to a ton in a footprint similar to that of a rack (1600 pounds on about 3 feet by 3 feet was reported on the list).
Another thing to remember is that you almost certainly want it to be twice the size that you think you ``need'' it to be at first. You need room for the physical nodes. You need room to walk around the physical nodes, attaching wires, punching switches, and generally looking busy if the boss comes buy. Assuming that you're not the boss, of course. Room for a desk or table capable of holding a monitor, an office chair, even a small workbench with a trusty electric screwdriver, multimeter, portable flashlight, and some bins for screws and whatnot is called for (and you should start scrounging for all of this stuff if you don't already have it handy). You need room to assemble and disassemble nodes, room to sit and work on node software via a workstation sitting on the desk, room to store various things. Finally, the robustness of your beowulf to failures in cooling depend mightily on how big the room is - how much ``thermal ballast'' does it provide. We'll talk about this further below.
Once you've gotten the size and the strength of the space worked out, you are by no means done. The next thing to work out is how to feed the beowulf what it needs to live and how to remove its waste products11.4.