One of the most common question asked about "units" in Goldsource is what real-world unit they correspond to. Unfortunately, 1 unit doesn't really equal anything. It is not a "real" inch or centimeter. It is just a figure. If you build your maps based on a unit actually representing a standard measurement, then Gordon will end up being a short fat person.
So what dimensions should I use?
These dimensions apply to Half-Life maps and other mods that don't have custom player movement.
The following figures are pretty good, but if you're making small spaces for players to fit through, it's best to add a few units just to be sure. A unit is the smallest square available in the 2D views. Just use your [ key to reduce the size of your grid until it won't go any smaller, then zoom in with the C key, and the grids should be one unit. You can verify this by checking the Snap on Grid indicator at the bottom right of the screen.
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 Standing: 18
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. Crouched: 37 Standing: 73
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
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
Note: Some users have reported that it is possible to crouch-jump 64 units high provided the surface you jump up to is sloping down towards you between 45° and 0°.
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
Note: These values are approximate, as jump distance can be extended using various movement techniques.
Looking directly ahead, a point will appear to be at eye level if it is 64 units above the floor.