Well in case you don't already know this, you should always use brush sizes of multiples of powers of 2. (eg. 64, 256, 384, 768 etc.) This will make brushes line up nicely with large grid sizes like 32 and 64 and you are infinitely less likely to get errors later on in your map when you start adding more detail.
And of course, if you do this, then it tends to be really easy to tell how big a brush is, because most corridors tend to be 256 units high and so on (notice how easilly a brush that size lines up with grid lines).
If you want to create a brush that is a similar size then you can just estimate roughly how big it should be. So for example, you could make the red brush 64 units long, but if it turns out you actually want it 32 units longer to align with some other piece of geometry, then you can easilly vertex edit it to be a bit longer (96 units).
This just generally makes your map much much easier to construct and these scales are tried and tested in the UT engine (for 5-6 years!).
After a bit of experience you can just glance at a piece of geometry and tell the dimensions just by looking at it. I could tell if a corridor was 256 units high or 192 units high with my eyes closed!
