Half-Life

Dimensions - by Andy

These dimensions apply to levels created with most editors, as long as the code for the player entities has not been modified.
If your using standard player models these dimensions apply.

What does 1 unit equal?
The answer is that 1 unit doesn't really equal anything. It is not a 'real' inch or centimeter. It is closer to a pixel.
If you build your maps based on a unit actually representing a standard measurement, then Gordon will end up being a vertically challenged rotund person.

Ground Obstacle Height:
This is the maximum height, in units, an object can be while still allowing the player to walk over  it.

Crouched: 18 units   Standing: 18 units

Overhead Obstacle Height:
This is the minimum distance, in units, that can be between the ground and an object while still allowing the player to pass underneath it.

Crouched: 36   Standing: 72

Minimum Path Width:
This is the minimum width, in units, that can be between two objects while still allowing the player to pass between them.

Width: 33

Jump Height:
This is the maximum height, in units, an object can be while still allowing the player to jump on top of it.

Crouched: 44    Walking: 44    Running crouch-jump: 62

Jump from - to:
This is the maximum distance, in units, that can be between two objects of the same height, while still allowing the player to jump from one object to the other.

Crouched: 99  Walking: 109  Running: 239  Running crouch-jump: 259

Eye Level:
Looking directly ahead, a brush will appear to be at eye level if it is about 45 - 50 units off the floor.

Other Dimensions:
The smallest brush you can walk on is A half of one unit... if it's there you can walk on it.

Damage recieved in a fall: (height in units)

Important: If a player falls in water he will not suffer damage!! Even if the water is only one unit deep.
If you require the Player to be damaged in a water fall, use a trigger_hurt entity with the spawnflag 'falling damage' activated.

Thanks to Tiger for the information.