Everyone knows that Hexagons are Bestagons, and that the 6-mile hex really is the only size that makes sense for wilderness travel. But since the dawn of RPG time, the 30-mile hex has also always been around and keeps showing up from time to time.
As someone who thinks that hexes are best used as a tool to approximate the length of a winding path between two points without having to fight with a measuring tape instead of treating it as a “wilderness room”, I always found the use of 6-mile hexes very compelling. Most wilderness travel will be something like 12 to 24 miles per day and you can easily set up a travel speed system where any overland movement will only be in full 6-mile hexes with no fractions and remainders. (And by you, I mean me.) Going smaller than that with the hexes becomes pointlessly granular, and bigger hexes become less useful for tracking daily travel. The 30-mile hex is way too big for travel tracking, and if you think the 6-mile hex is ridiculously big to hide just one encounter, then 30 miles is just ludicrous.
However, I was once again struggling with frustration about not having a clear image of how I want to handle the contrast between wilderness and civilization in the Kaendor sandbox I am still working on. And it occurred to me that perhaps I could make the city states much smaller and treat them as being on the same scale as individual barbarian tribes that live spread out over several villages in a limited area. And I think the 30-mile hex might actually be a really good unit for the territory claimed and mostly controlled by a mid-sized town or a tribe.
A 30-mile hex with the main settlement in the center means an area with a radius of 15 miles. That’s about the distance that you can travel with cargo in a day in pre-modern times. (Though of course express messengers can go much further than that.) This allows people from the outer edges of the area to travel to the central main settlement in a day, stay for the night, do their business in the morning, and make it back home before nightfall. Historically, towns organically grew to be spread out at half that radius for their respective area of influence so people could make it back home on the same day. But that’s for medieval Europe or the early American colonies. For a sparsely populated setting and in a frontier context, I think 30 miles should be very suitable. (In a more densely populated and developed setting, 10-mile hexes could be very useful too, though.)
I think that a 30-mile hex also makes for a good size for a forest or swamp in a sandbox. Each 30-mile hex contains 18 6-mile hexes and 12 half-hexes. Assigning 24 hexes to a geographic region with shared environmental conditions and using the same wandering monsters tables seems like a pretty good size if the campaign is about traveling to spread out ruins instead of clearing hexes where every hex contains a thing.