This rule comes from the early days, where FPS players lept and bounded like strange, violent gazelles. Our younger readers may not know how different the early games were; for example, the running speed of the original Doom Guy in real terms is said to be about 60 mph. He could keep pace with a rocket fired parallel to him.
With so much play centered around mobility/maneuverability, a fun level was one that gave you enough space to work, but not so big as to be slack. You need room to circle-strafe, clearances to rocket-jump to. We may not be doing much of that in modern FPSes, but it's still important as ever to give the player room.
Recall the refrigerator box: the player is more unwieldy and blinkered than he thinks he is. Because of this, the level designer is called upon to build spaces around him that appear realistically scaled, but also tailored to mask the player's shortcomings. The simplest formulation of this design maxim? Keep It Wide.
This is to say, give the player room to maneuver/direct fire, and give enough space for clear visual navigation. A necessary corollary to this: the more players that play together in a space, the more room is necessary. Like all design rules, there are good reasons to violate this rule which we'll get to, but consider it good general practice.
Let us consider two scenarios--one from Valve's own Left 4 Dead campaign No Mercy, the other an excellent work in progress custom campaign, Highway to Hell.
Both campaigns begin in an apartment complex, but there are some crucial differences.
No Mercy: Bill is puzzled that the apartment's hallways are so generously sized, but knows he's got room to maneuver.
Highway to Hell: the survivors tend to see more of each other than zombies in some hallways. No place for a shotgun.
The setting and quality of set dressing are nearly identical, layouts very similar. One of the few small differences is that in Highway to Hell's apartment building, there are some extremely tight spaces for 4 players to fight through effectively. In L4D all players can freely clip through each other, which eliminates a lot of common related issues of this kind, but not all--friendly fire is still dealt. Which is exacerbated if players don't know the common practice of crouching if out in front.
No Mercy: Maybe a little oversized for the kind of mid-level apartment it would appear to be, but it gives room to play. No one joyfully shouts "this is so realistic!" when they're not having fun. (Outside of ArmA2 players, of course.)Play through the beginning of No Mercy, paying attention to horizontal space. Most areas are built to accompany at least two abreast at any point--meaning if the two forward most players kneel, this is clearance enough for all four players to fire forward.
Not only does it make the game's stated emphasis on co-operative play viable, but it makes navigation easier as well--the more spatially restrictive an area is, often the harder it can be to clearly navigate. (In another update, I'll elaborate specifically on navigation considerations.)
The above shot is perhaps the diciest example from Highway to Hell. Most of the way through the map the survivors will wind their way through a meticulously recreated gas station.
The sense of place is very real, possibly to a fault: this communicating hallway is narrow and has a whopping six doors connected to it--to the front of the store, the back door, the break room, the restroom, and so on. While this is laid out realistically, suggesting it was all modeled on an actual gas station, it's also a death trap.
It's deceptively hard to move around in and direct fire, and if the Director throws down on the players through one of these connecting rooms--as it is wont to--the players might have a very hard time escaping, much less as a group.
I haven't had the pleasure of playing this map in actual co-operative play, only with bots, so I can't speak to whether the experience of it is panic inducing or annoying. The rub is that these two kinds of experiences live in very close proximity to each other.
This brings me to exceptions to the rule. Valve violates "Keep It Wide," that they otherwise strictly observe, in several key points throughout L4D. There is a storm drain sequence where it's obvious the players will only be able to advance in single file, a dangerous proposition in a world of zombies.
The above communicating hallway layout from Highway to Hell may also be an exception: despite being uncomfortably narrow to move and direct fire in, it is also small, and well connected, and veteran players should be alert enough in such a tight space. (Designing with the 360 version in mind would mean this area is simply out of the question, however.)
The more I think about it, the more I'm inclined to think it is an exception. Though I would note that the navigational issues (difficult to orient in such a nondescript corridor with so many doors) would be helped by introducing orienting details: restroom decal and possibly some chinsy art hanging on the opposite wall.
But the truth is, like every design decision made, you won't know until you get playtesters. If this area is consistently panicking players or making for some tense little shootouts, it's worth keeping, but if it's producing disproportionate casualties or an untoward difficulty spike, then it's worth revising.
Any design decision that violates best practices for level design in your game must be carefully considered. Is it breaking a rule in a fun, tension-inducing way, or is it doing it in a completely unfair, obnoxious way? You won't know without playtesting.
But rest assured that you'll address many aggravating issues with playtesting before they start if you remember "Keep It Wide."
[This was by no means intended to pick on the designer "Unlawful Combatant." Highway to Hell is absolutely professional quality work, and once its polished and completed it will rival or surpass a number of the original campaigns in the game; readers with L4D should check it out and see just how many of the previously mentioned Level Design Primer concepts they can spot (hint: basically all of them).]