Ideas like this actually came up frequently as common alternatives to the Volume and Mass suggestion I was pushing for earlier.
What it comes down to, however, is that for either this or Volume and Mass's idea of precise units but fungible sizes of specific pieces of material, you need stacking first.
The reason we only get one log or one boulder now is that large item lists bloat the current item vector (although I do wish DF would move off of a vector and onto a container type that is more functional with large numbers of contained pointers) is that we simply cannot stack 10 wood planks as a single item times ten, we have to store them as ten individual items.
It's less a problem now that dwarves can collect from multiple piles, but the primary reason that this sort of suggestion wasn't acted upon before was that dwarves needed to collect items from multiple piles if they needed 5 units of wood planks or something. Without stacking, this is still a problem.
Mining: for mining, I think that a stone taking up 1/3rd of a tile might actually be easier (so 3 rubble or 2 rubble and one boulder with a possible giant boulder taking up 2 regular boulder's worth) than 7, and 7 is just the number we are most familiar with.
For these, rubble (or gravel or whatever you want to call it) would best be handled with a generic non-ore rubble type and be capable of stacking with the rubble.
People have pointed out that ores should often be a type of gravel or rubble rather than a boulder, as well, which would be a typed distinctly (hematite rubble/gravel) before being worked.
Sand is something we may want to consider more carefully. While I'm all for having a fixed definition of how much sand is allowed in a tile, and making pickaxe mining a Non-Newtonian Fluid impossible, a blunt 7 units of sand per tile is exactly the sort of problem with item sizes that we're supposed to be moving away from, here.
The same goes for wood - part of the objective is to make trees of different sizes, which means more or less wood per tree, rather than a one-size-fits-all 7 wood. This is especially important if we have tree farming later on with improved farming, and we have different "crop yields" in the tree farming.
Anyway, this whole thing came up pretty often in some flavor or another in the actual Volume and Mass thread, and I'll say here what I said there:
So long as there is stacking, there is NO FPS difference between having the game track "10 liters" or "7 kilograms" of wood and the game tracking "wood [5]". Literally the only difference between a stack of one and a stack of the other is that in Volume and Mass, the units have a distinct name.
For example, currently, metal ingots take up 2 liters. So, a stack of "Iron bars [4]" is exactly the same thing as "Iron bars: 8 liters". (Excepting, of course, that in this case, the unit of measure is half that of the other, but as long as stacks are nothing more than integers of a given unit size, that's largely irrelevant.)
So, basically, your "simpler" solution is actually no simpler at all. The Volume and Mass idea is essentially the same thing, but with a name for the units that makes comparisons between different stacks easier.
EDIT:
In fact, the common unit of measurement in the game (at least, the raws) is the cubic centimeter. That is the unit used in [SIZE:whatever] in the creature raws. It is probably also what is used in the hard-coded raws. SIZE, which everything has defined, is just a cubic centimeter.
If we go by my calculations of a tile, we have [SIZE:27000000] in a tile. Toady recently talked about a 2m*2m*3m tile for minecart physics, so that would translate into [SIZE:12000000] tiles. If you just declare that a tile full of sand has 12000000 cubic centimeters of sand in it, you're pretty much done. Again, this is the unit of measure the game already uses for everything, anyway, it just doesn't tell you outright yet.
You just need to use a unit to display that volume, and liters (SIZE:1000, or 12000 liters per tile) would be an easier way of doing so. Then, you can compare a liter of meat to a liter of wood to a liter of metal.