Building on the grid is so important I dedicated a page to it, even though there is very little content.
Just a brief examination of the sample maps will uncover a simple truth. Professional map builders build on the grid. It establishes a standardization, which helps make the world appear consistent. To that end, world scale is very important.
If a average player character is 96 units high, that door is around 128
The clear advantage to building on the grid is your textures tend to align properly. Perhaps not a hard and fast rule, most textures are a power of 2.
Have a look at these brushes, each a standard on the grid construct. If you notice nothing else, notice that the textures are properly aligned. And that required absolutely no additional work. If that's not enough to convince you, well, then, trust me, in six months you'll thank me, ignore this, and suffer at your own peril.
World Scale
I stumbled on the official Monolith scale, simply by taking my extensive experience in building in the real world and extrapolated it for Dedit. Since Monolith never released this information it required a post in the NOLF official forum to confirm my assumption.
This a copy from a post I placed on the official forum a little while back when people were asking what the relation between Dedit and the real world is.
I'm including it here because there have always been ramblings from mappers trying to figure out the scale in regards to Dedit. This is it.
Subject: scale
Author: brazen <m0dmaker@aol.com>
Date: 10/25/2001 11:09:28 AM
I always thought 64 units was about 4 ft, 128 units 8 ft, 256 unit 16 ft. All my maps are built using that as a guide. it might be wrong, but visually it feels right. The basic player model is 96 units which is approx 6 feet. Whatever measure of scale you decide to use, always keep it simple, and consistent, a measure of 16 has been used in the building trades for over two hundred years.
Author: brazen <m0dmaker@aol.com> Date: 10/25/2001 11:12:10 AM
Just to clarify 16 Dedit units = 12 inches = 1 foot
Author: Chris Hedberg <hedberg@lithtech.com>
Date: 10/25/2001 11:53:27 AMSubject: RE: Dedit Units
Yah, Brazen's got the "official" scale. In reality, if it looks right and your characters can navigate it properly, it's the right scale. :) In Shogo, where you piloted a giant robot, 16 GU was probably 3 meters. :) It is, as a great man once said, all relative.
-Chris
Chris Hedberg is one of the LITHTECH gurus employeed at LITH (the Monolith Subsidiary).
SO WHAT'S THAT MEAN?
Basically you can extrapolate standard sizes from those offical numbers.
basic characters = 96 Dedit units = 6 feet
basic rule, always use the 16, 32, 64, 128, 256, unit grid, it will help align your textures so they always fit right.
pred jump is around 296, with the absolute max being 300. But a 288 (256 + 32) ledge should give the right level
basic stair 16 units = 1 foot
counters, tables and window heights 48 - 64 = 3 feet and 4 feet respectively
door height at least 128 units, but a lot of them are 192, to accomodate Queens, Preatorians and Exosuits.
WORLD SCALE
8 units = 6 inches
16 units = 1 foot
32 units = 2 ft.
64 units = 4 ft.
128 Dedit Units = 8 feet
256 Dedit units = 16 feet
512 units = 32 ft.
Marine/Human crouch is 48, so ducts 64 x 64 are a tight fit. Openings that are at least 128 x128 can be accessed by all Deathmatch models ( think of the vents in Colony).
Basic human model can fit in a corridor/crevice space 32 unit wide. An Alien needs a minimum of 48. Hallways and vents should be a minimum of 128 wide, which would be about 8 feet.
One thing to note about sizes, the FIRST PERSON, makes everything look 3/4 smaller. So a handrail that looks right at waist level in FPS view, will look chest height relative to third person view.

Due to the angle of the shot, it might not be clear that the height of the Preatorian is 128 units, and the human models are in the 96 unit range, the drone is hunched over by the particular animation, but when standing or running it is about the same height as the Predator which is about a head taller than the marine.
Sometimes mapping is a question of what feels right, nonetheless, having an official scale gives you a standardization, that can be consistent from map to map, and between level designers as well. If everyone is building a standard 128 or 192 high door, then texturing for it is easier, a standard move amount can be agreed upon, and a systemic constant can be achieved. Even if you don't understand what I just said, you should remain cognisant of the scale.
One other thing to note. This room is built on the grid. It is 256 units high, 256 units wide, and 512 units long. Notice anything? The wall textures fit perfectly. And yes those are LAST ONE OUT characters.