The red or whitel cirle indicates how far out the dot dar will appear on your clipboard map. Enemy planes within that circle will appear as a dot and are updated every ?? seconds (I think 10 or 15 seconds is the current setting). Friendly planes will always appear as long as your HQ has radar enabled.
These are for the MA. The special events arena have different values and they are usually posted on the MOTD.
As for looking out the window, the MA norms are split into 4 settings:
1st setting enemy fighter above 500 AGL is shown to an icon range of 6k.
2nd setting enemy figher below 500 AGL is shown out to an icon range of 3.5.
3rd ground vehicles are only shown at 1.5k when in a plane.
4th ground to ground: when in a vehicle, no icons are displayed for GVs, but #1 & #2 apply to aircraft.
The exception to #4 is the PT boats who show up at 3.0 when in a GV.
I believe there is an exception to the above for LVTs also. One of the trainers should be able to answer that.
Hope that helps.
Again the SEA has different settings. The CT may also have different icon settings.