I honestly don't know that Groundscale has ever been officially 'set' in NetEpic;
(it may have done in EA or E3).
But: After a bit of rumination,
I think I have visualized it at about 1Decimetre = 1Kilometr,
so 1 meter is 10 Kilmetres or 6.21371 miles...
...that makes a 4'x6'  table is  7.57556miles x 11.3636 miles,
(or  ≈8x12 miles) ,
and a 6'x8' table would be 11.3636 miles by 15.1477 m
(or  ≈12x15 miles)
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Or, more simply:
 6" ≈ 1 Mile... ÂÂ
By no means official, but:
It has the benefit of metric conversion tables and math &stuff,
more importantly; it 'feels' about right, So;
works for me...
...>FWIW>IMO>YMMV<
Of course, it needs must be said:
My sense of scale comes about from having been weaned on Rogue Trader,
when weapons with 72"+ ranges were the norm.
These days: 40K  weapon ranges are greatly foreshortened:  6"-12", so...
...A good argument could be made that NetEpic therefore
has a groundscale of approximately
1 city block= 1 foot
http://en.wikipedia.org/wiki/City_block,
so a 4' x 6' table would be 1/3 x 2/3 mile....
:
One could also argue that groundscale is flexible and indeterminate,
similair to Flames of War,
and combine the two scales above depending on the terrain layout....