Combat frontage and dungeon design in D&D

Want to make your D&D combat on a 5' grid feel a bit more like OD&D? Make your basic hallway map square a spacious 15' by 15', translating to 3 combat squares across, or assume three guys fit in 10 feet if playing theater of the mind. Why? Well, back in the day, moving on from playing Chainmail to D&D in early '75,  we packed three guys into line to fill a rank in a 10' hallway, based on historical close order infantry like Romans and Anglo Saxon shieldwalls. One 10' by 10' square held nine, not four guys. Two guy could space a bit open order or split the gaps to the walls on either side and hold a hallway without being flanked, since there was no "snap to grid" of 3rd ed and later, but three could pack in and fight as the first rank.

This makes a bit of a difference in dungeon crawls, since except in real chokepoints like doorways, bigger parties didn't become such straggle-fests of characters with their line of sight to the enemy blocked. A typical optimized heavy melee front of PCs and hireling warriors was three with weapon and shield in front and three more with pole weapons or missiles or casters in the second rank.

The 5' grid of later standardization is for the convenience of miniatures play with big single bases after scale creep had driven up the size of figures to 28mm and beyond, often on 1" circle or square bases. Three dinky little 25mm Minifigs or McEwans on close order basing will fit a 10' passage on those maps as long as you aren't sticky-grid about it.

Does it matter? A bit. Besides more guys obviously getting to do something in a basic hallway, and more possibilities for flanking in a hallway fight, if you don't do 15' but do 10' is three squares, fireballs and other area effects are even more lethal, when over twice as many slavering orcs can pack in and be blasted by one.