The worst possible thing to happen to any mapper is to map out a large part of your map only to realize that it's over expansive or too small. This is where size references come to great use.
For starters, I highly recommend staying on Hammer's grid. You can use the "[" and "]" to change your grid size. Staying on the grid pretty much reduces the amount of errors you may receive and the Hammer Editor compiler will love you too. Along with reducing errors, it makes your mapping much more neat and precise.
As for size references, I created a reference vmf where I'll be storing various references for my map. Be it styles, size references, dev textures etc.
I placed some prop_static of various Left 4 Dead characters. This gives me a good idea of size reference of a area. Likewise, you can use the spawn point entities for the same purpose.
"Size Reference Group Photo"
Here's another good size reference. Cars. We all have some sort of grasp how cars measure in real life, that way we can use this method to make better judgments of measurements in our map compared to reality. Likewise, in this method, survivors can be placed to make even finer measurements within the cars. Nick represents every car mark, and Ellis represents every halfway/inbetween car mark.
"This fits with that "On the Grid" thing mentioned earlier"
Dev textures are a good size reference aswell. If you've randomly joined a custom map server in any source game, you have no doubt come across a dev texture map. The good thing about the Left 4 Dead 2 Authoring Tools is that they have changed the dev textures to something more eye pleasing.
"Old Method: OH WHAT THE- MY EYES"
As you can see, they're different shades of grey, rather than blinding orange. The shades of grey represent different reflectivity in regards to radiosity. Remember that the source engine is capable of radiosity. Radiosity is allowing the color and light of a texture to bounce off due to lighting. If you have a green wall, when light hits it, it'll reflect green back to it's surrounding environment. You can use these dev textures to represent various shades of textures you might use later on to take advantage of radiosity. For example, if you want a bright plaster wall, you may want to use a dev texture with higher reflectivity while for a darker gravel road you might want to use a darker reflectivity amount.
The wall dev textures, when using the default scaling (0.25 by 0.25) represent a good height of one story.
"2 Story Building, looks about right"
Keep in mind that this is just a general measurement, you have to take in account actual architecture for your measurements. For example, Southern architecture have higher ceilings because of the heat. You can see this represented well in Left 4 Dead 2. In cases like this, you may want to create "half-sized" dev Textured walls to help create additional halfway marks or whatever measurement you want to add. Think of the default 0.25x0.25 dev texture wall as one unit. Just multiply or divide 0.25 by what measurement you want.
Here you can see my half-sized dev texture walls combined with the normal default sized dev textured walls. The half-sized dev textures are scaled by 0.125x0.125, or when simplified 0.13x0.13. (0.125 was made by dividing 0.25 into half.).
Hope this was useful to any mappers out there. If you have any other useful advice/tips you'd like to share feel free to email them to me and I'll include them in a post with your credit.
My next blog post will cover blocking out the first map. Til then.