The two keyvalues that you'll need to know is maxs and mins, MINS meaning Minimum Size and MAXS meaning Maximum Size.
"MAXS" is for the positive direction that you this point will move in relative to its origin. The numbers for this keyvalue can ONLY be positive or it will not work.
"MINS" is for the negative direction that your second point will move in. Values can only be negative.
A Trigger can only be seen with the console command "Picker" and it will draw the entity under your crosshair with 3 lines
GREEN being X,
RED being Y and
BLUE Being Z Vector or Axis. Were the lines converge is the Point of the entity.

Here is an example of what a 128 x 128 box would look like.
maxs "64 64 64"
mins "-64 -64 -64"Once you set the MINS and MAXS picker will now show the volume of the entity.

You can set the MINS and MAXS on most Function entities and all triggers, Basicly anything that needs its volume set to work. you can also make triggers in any size the one i made was a perfect box you can make a rectangle or ect...