I'm building up a hexcrawl campaign, and I've had some thoughts on scale. Scale has a number of issues, many of which have been touched on here, and you need to answer some questions about what you want out of your hexmap to determine a good scale fit.
(A note about hexagons and measurements. When I refer to the size of a hexagon, I'm referring to it's 'flat-to-flat' size, ie, how from from the center of a face to the center of the opposing face. This affects how you determine the area of a hex, which I find useful in considering scale. A 1 mile hex has an area of about 0.87 sq miles, a 6 mile hex has an area of about 32 sq miles, and so on. Area can be estimated by using 3.464 x sqrt (d/2), where d is your flat-to-flat distance. If you use a measurement of hexes for each side of the hex, they get much bigger (a 6 mile on a side hex has an area of roughly 77 sq miles, frex).
To begin, you need to decide how much realism in exploration you want to have. I'm not talking about survival here, but actual exploration. Being flat, hexmaps don't provide the rich views that actually traveling across landscape provides, but taking those things into account can add extra complexity that may not suit your intent. Sight distance is such a factor. A good rule of thumb is that you can see (or be seen) 1.2 miles x sqrt(height). This means that your average adventuring party, on flat ground, can see between 2 and 3 miles away (halfling to 6' human). Making a hex much bigger than twice this means that a party traveling through even a completely flat and clear hex could not see all of the features in the hex. Add woods or rolling terrain and this gets much, much worse. Further, players should be able to see mountains and other tall landmarks from far away (a 10,000' mountain range is visible ~ 120 miles away. A good hill (150' above surrounding terrain) extends sightlines to ~15 miles. This can add a great deal of extra complications to a hexcrawl, as a party may be able to see much more of the map than the hex they are in or, alternatively, much, much less than the hex they are in, if you're taking into account more realism in sight.
This leads to the question of whether or not such additional realism is worth it. I'm taking a hybridized version, where I apply a simplified sight radius to hexes and features. Firstly, to be clear, I'm working in 1 mile hexes (this would changed based on bigger hexes). Each terrain type has an associated number assigned, with forested hexes being zero, plains and similar clear terrain being 1, hills being 7, and mountains being 50. Being in a hex allows you to see into surrounding hexes of a lesser rating up to a distance equal to the rating. (These are smaller than reality because I'd rather cheat on the side of less revelation than more.) Example: standing in a forest hex means you cannot 'see' into adjacent hexes. Standing on a hills hex would allow you to 'see' over up to 7 plains or forest hexes distance, but not past an adjacent hills hex. A forested hills hex would be the same as a forest hex. Standing on a mountain hex would give a tremendous field of view (and a good reason to maybe climb those mountains). You can see points of possible interest in adjacent hexes that you could see into with an appropriate check for those features (example, a ruined keep in an adjacent plains hex would be immediately visible, but a goblin cave hidden among some scrub brush might have a DC 15 or high perception attached to it. Otherwise, having visibility to a hex would only reveal it's dominate terrain features (hills, forests, etc.).
Some terrain features may have an associated visibility rating as well, such as a 100' tall tower with a rating of 5, meaning it can be seen from up to 5 hexes away.
The second issue with considering what scale you want to work with is how you plan to use your map. If your concept is exploration with a focus on thoroughly exploring an area, say to pacify a region for settlement or if small resolution features such as caves are important, then I'd recommend going with a smaller scale like 1 mile hexes. If you're mostly using it for overland travel, or for exploration between fixed points that need to be discovered, then a large scale is appropriate. Generally, if using a larger scale, you need to abstract out many things and just handwave the idea that they can find the bits you want them to in a 30 sq mile area by walking through it at a normal pace once. I've done this in the past, and it's perfectly valid to do this, but you should recognize the reduction in realism. If your plan is for a gritty hexcrawl, that may be too much reduction for your needs.
On the other hand, small scales can be quickly traversed, meaning you need to extend your maps while larger hexes won't require this. Still, a 6 mile hex leaves a lot of space, and even big D&D apex predator types rarely need a 30 sq mile territory to live well, so if you go with a larger hex size, don't be afraid of packing many or even most of the hexes with interesting things. For example, the typical wolf pack has an average territory of about 15 sq miles. That's enough room for two packs in a single 6 miles hex. Wolves also claim a far larger territory than they need, so they make a good rule of thumb for larger predator types. Figure about 10 to 20 sq miles for active hunting animals and about a 1/3 of that for more intelligent creatures to about that for small tribes. A troll may be perfectly happy with about a 5 sq mile range, but a goblin tribe probably needs about as much area as a wolf pack. So don't be afraid to pack in interest into larger hexmap scales.
