The Half-Life unit to Real Life unit ratio has always been a hot topic of discussion. First off, let me get one thing straight: there is no ratio, period. It is completely impossible to find a perfectly proportional relationship between Half-Life units and real world units. Why might you ask? There are several reasons.
Usually when someone attempts to do this conversion they end up with 1 inch equaling 1 Half-Life unit. The logic behind their thinking simple - a player is 72 units in height, and the average human is 6 feet tall, or 72 inches. It doesn't take a genius to calculate 1 inch = 1 unit. But wait, the player is also 32 Half-Life units wide... I don?t know about you, but last I knew Gordon Freeman wasn't nearly 3 feet wide.
Ok, even so, you say 1 inch = 1 unit? but try to actually build
a map using this ratio. I guarantee you things will look very odd and disproportionate. Things like doors and doorways will look especially strange.
The Half-Life engine was simply not
built with real life scale in mind. In actuality, 1 Half-Life unit equals 1 texture pixel. All the textures were designed to look in-scale with the player's vision (which doesn't have the same view cone as a normal human). If you want to make something in scale, you won't be doing it in inches, meters, or any other real life measurement.
You can still scale things to how they appear to the player, however. Most doors are generally 64 to 96 units wide, and 96 to 128 units tall. Hallways are anywhere from 64 to 256 units wide, and 96 to 192 units tall, just depends on what
your trying to map. Stairs should almost always go up by 8 units each step, 16 is far too much and just makes the stairs look, well bad. Stair width is also good to keep low, usually 12 or 16, 32 is just too wide and your stairs will be extremely long, and 8 is too compact and claustrophobic. Using an info_player_start entity for scaling is very useful, as is the Half-Life Dimensions
article. Knowing these limits will help you further understand what the player is able to do within the level and how he or she can navigate obstacles.
Textures also help scale the size of rooms and corridors. Most are 128 x 128 units, and that is also the general height of a hallway or room. Of course you can always scale a texture to make it fit in a larger area, but you'll loose quality. Doors are also meant to be kept at a 1-to-1 scale, and are usually 64 x 96 units in size. Trying to apply these textures with a real life scale, like 1 inch = 1 unit, will just result in odd looking, malformed textures.