Maybe something to think about (WDS or another publisher?)

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Usually I have started with eyeballing a map; I did this for Virginia to work out some distances from around Norfolk past Richmond to check the different amounts of hexes required at some different scales. More or less the area that I was looking at was about 150 miles long - and I think I was looking at 200 meters per hex -so basically about 8 per mile. 

Of course distance is one thing, filling in the details in between are another; but knowing the period and the approx scale, (and period) - I doubt that is going to be a significant problem.

For what it's worth, I don't actually think that Civil War has to necessarily use the Civil War engine- mainly because what is being talked about is making a different scale; in that regard what you are looking at are various features that can be transferred to the larger scale. 

Yeh sure combat requires some modification; and there are some features that are not necessary (flank, rear impact) - that's stuff that you hammer out within an engine.

I wasn't posting specifically in reply to anyone - so I mean, you've outlined some of what would needs to be done Ian, sure thing, and you're entirely correct.

I don't know, that I would use an existing map as anything other than what dimensions have been used in the past. As an estimate -probably would be just as useful to grab one of those existing boardgame maps and then get an idea of the hex dimensions you are talking about.  I started on something not as large of a scale (roughly the scale of GMT's Battles of the American Revolution) and added all of Narragansett Bay to it. Thing is, though, I don't have any details about roads around the Bay - Aquidneck Island's maps have detailed road nets (at LoC).

Edited: the attachment give an example of something done at a larger scale (and it's still 8x smaller than the proposed - and nevermind it is also for the wrong period).