Yeah, the hexes don't have any particular size. Worse, it's not a matter of how far the weapons can shoot (the 105mm certainly has more range than the 25mm) but how at what range can they have the relative effectiveness, taking ROF into account. At the game's level of abstraction the identical weapons ranges work.
The 105mm has a blast radius smaller than the 500 and 1000lb bombs in the game; if the latter don't get [x], the former probably shouldn't either. Having a sizable area of effect is necessary to take out a whole unit of infantry, who are presumably spread out a bit. The weapons that can only attack point targets (vehicles and buildings) have a special trait to indicate that. The cluster bombs in the Mk 20 scatter over a big area, with full effect on a piece of real estate at least as big as a football field. And then you have the MLRS (card), which attacks every hex on the board. It's been referred to as the "grid square removal device", blanketing a square kilometer. That helps to pin down the scale some; based on that I'd guess the hexes are supposed to be in the low hundreds of yards across.
I wouldn't compare the blast radius of a 105mm shell to that of a 500lb bomb. The AC-130 typically carries 96 rounds of 105mm; if you spread that over 10 turns that's about 10 rounds per turn. Unlike the 247 bomblets in a Mark 20 the 10 rounds can be individually targeted. If the AC-130 only fires in 6 of the 10 turns, that's 16 rounds/turn. Also, some turns it will get it's target within the first couple of shots, freeing more rounds of ammo for other turns. So I definitely wouldn't think of an AC-130 attack as being a single shot of ammo in any sense.