If you run vbspinfo on your map file with the -size command it'll output the bounds of your level like so:
If you take the "No Skybox" Measurements and define the first vector as "WorldMin" and the 2nd vector as "WorldMax" you can get the correct scale and position to take the overview from as follows:
ScreenWidth = 1280
ScreenHeight = 1024
Scale_X = (Abs(WorldMin[0]) + Abs(WorldMax[0]))/ScreenWidth
Scale_Y = (Abs(worldMin[1]) + Abs(worldMax[1]))/ScreenHeight
If Scale_X > Scale_Y then
Scale = Scale_X
Else
Scale = Scale_Y
CenterPos[0] = ((Abs(worldMin[0]) + Abs(worldMax[0]))/2.0) + worldMin[0]
CenterPos[1] = ((Abs(worldMin[1]) + Abs(worldMax[1]))/2.0) + worldMin[1]
CenterPos[2] = worldMax[2] + 2000
Then open the map in game and spawn as whatever team and do "setpos_exact CenterPos[0], CenterPos[1], CenterPos[2]" and "cl_leveloverview Scale"
Note that CenterPos[2] is just a guideline value, you can set it to pretty much whatever you want, it'll just change what actually appears in the overview.
If you want to take a super high res photo, you could hypothetically subdivide the above calculations into a uniform grid without too much difficulty.
***
Or just use this quick calculator I hacked together:
[ParseHTML]
<div id="geit_CalcForm">
<label for="geit_ScreenHeight">ScreenHeight</label>
<input type="text" name="geit_ScreenHeight" id="geit_ScreenHeight" value="1024"/><br />
<label for="geit_ScreenWidth">ScreenWidth</label>
<input type="text" name="geit_ScreenWidth" id="geit_ScreenWidth" value="1280"/><br /> <br />
<label for="geit_WorldMin0">WorldMin</label>
<input type="text" name="geit_WorldMin0" id="geit_WorldMin0" />
<input type="text" name="geit_WorldMin1" id="geit_WorldMin1" />
<input type="text" name="geit_WorldMin2" id="geit_WorldMin2" /><br />
<label for="geit_WorldMax0">WorldMax</label>
<input type="text" name="geit_WorldMax0" id="geit_WorldMax0" />
<input type="text" name="geit_WorldMax1" id="geit_WorldMax1" />
<input type="text" name="geit_WorldMax2" id="geit_WorldMax2" />
</div>
<br/>
<p id="geit_SetPos"> setpos_exact <span id="geit_SetPos0">0</span> <span id="geit_SetPos1">0</span> <span id="geit_SetPos2">0</span></p>
<p id="geit_Scale"> cl_leveloverview <span id="geit_Scale0">0</span></p>
<script>
$(document).ready(function(){
$("#geit_CalcForm input").change(function(){
var scale_x = (Math.abs($('#geit_WorldMin0').val()*1) + Math.abs($('#geit_WorldMax0').val()*1))/$('#geit_ScreenWidth').val()*1;
var scale_y = (Math.abs($('#geit_WorldMin1').val()*1) + Math.abs($('#geit_WorldMax1').val()*1))/$('#geit_ScreenHeight').val()*1;
var scale = scale_x > scale_y ? scale_x : scale_y;
$('#geit_SetPos0').html( (Math.abs($('#geit_WorldMin0').val()*1) + Math.abs($('#geit_WorldMax0').val()*1))/2.0 + $('#geit_WorldMin0').val()*1 );
$('#geit_SetPos1').html( (Math.abs($('#geit_WorldMin1').val()*1) + Math.abs($('#geit_WorldMax1').val()*1))/2.0 + $('#geit_WorldMin1').val()*1 );
$('#geit_SetPos2').html( $('#geit_WorldMax2').val()*1 + 2000 );
$('#geit_Scale0').html( scale );
console.log(scale);
});
});
</script>
[/ParseHTML]