Out of spare time, I started to wonder how much water is in one "unit" of water, and thus how much fills a tile. Since a water unit is equal to the amount of water needed for one dwarf for one month, and they probably have similar water usage properties as a human, a 180-lb. dwarf (shorter but stouter and denser) should use 42 gallons of water a month. That's 350 lbs, and 5.6 Cubic Feet per 1 unit of water. Finally, 5.6 * 7 = 39.2 Cu. Ft. Thus, each tile is just under 40 Cubic Feet of space.
Now, if the tiles are 2.5 feet square, roughly the size of a normal doorway, or the narrowest a cave passage can be and still be navigable, that means that it's 6.25 ft. square and thus 6.272 ft. Tall. This makes sense, as humans can comfortably move in dwarven passages without crawling, and dwarves wouldn't settle for just small passages when they could make them full-sized.
2.5 feet in either direction allows you room to stand when fighting, but forces anyone else within that area to squeeze or crouch past you. It is also where the "Center Point" is for larger creatures, whom may have extensions such as wings or tails or claws past the edge, but if you want to hit the core of the creature, you must aim for that tile. That said, I have no mathematical proof that 2.5 is the right size, so annother number may be more useful.
A last statement; I think that tiles are not exact sizes; in particular, walls are not necessarily 2.5 feet thick, and can add or subtract a foot per side, easily. This means that your 2*6 bedrooms are between 3" by 13" and 7" by 17", but most often 5" by 15", and that is just about enough room for a bed, a cabinet, and a chest, as well as some walking space.
[ May 22, 2008: Message edited by: PTTG?? ]