Use feature props that you recognise, they allow you to gage distances through their size.
For example, when designing the middle area of my CTF contest entry (sabotage), i dropped the 2fort bridge "chasis" down to make sure my area wasn't too big/small.
Additionally, use sentry's like the player models as Midget said. Their sight range is 1100, so set their fade distance to that, when you select them you get a ring around them that shows their firing range. Anyone who plays engineer or finds themselves taking a lot of sentry's out (soldier/demo players in particular) will figure out the defencive dynamics of an area that way as well. Generally areas are slightly larger than the sentry sight range, depending on the map. So 1024 - 2048 are good area sizes.