Of course, there is no ideal hex size or campaign map size. There's only finding the right fit for your campaign. If world travel is a major goal of your campaign, by all means break out the 24, 30 or 36 miles hexes. Personally, I'm thinking that 5 miles per hex is too big for my needs. According to Wolfram Alpha a five mile hex is half a Manhattan, or one third of Walt Disney World (not just the Magic Kingdom, mind you, the whole dang operation). If my math is right, a 5 mile across hex encloses 16.24 square miles. Any hexcrawl campaign that posits only one thing in such a space is letting the artificiality of the hexagon do some of the thinking for it. Which is okay, that is why we use simplifications like hexes. I'm just for consciously considering the ramifications of such a choice.
Bob Bledsaw and crew knew that although 5 mile hexes might sound small in the age of the automobile, they are actually big enough you can get lost in. Here's a favorite bit of mine from page 38 of the classic Judges Guild supplement, Ready Ref Sheets:
"When entering a hex containing a village, tower or castle, a 6 on a six-sided die indicates that the feature in question has actually been found, a 5 indicating that a small farm or hamlet (10-60 population) has been found instead. Players following a road, coastline or river that intersects a village negates the necessity of 'encountering' same." (p38)Elsewhere in the Wilderlands material is a note that any five mile hex contains 0-5 additional items not in the key, but I can't find it at the moment.
I run dungeons mostly, so the campaign world exists primarily as the context for the dungeon adventures. My new campaign map is based on a model of one league per hex. We don't use leagues very much any more, but one way of defining them is the distance a man can walk in a hour, roughly 3 miles. Of course, someone in chainmail might need 90 minutes, should you wish to make use of those sorts of rules. Someone on a riding horse needs only 30 minutes to cross the same hex. Something like this:
Warhorse or mule
1 ½ hours
1 ½ hours
2 hours 15 minutes
1 ½ hours
1 hour 20 minutes
Of course, this chart basically comes down to one hour per hex, plus a few simple modifiers.
Basically, I want a game world where getting to the dungeon is a good, refreshing hike and travel times to cities and castles can be measured in hours or at most a few days. So I'm scaling my world accordingly.