Some items already get a size, namely bars of metal and soap (and ash I guess). In fact, the soap bars broke the metal bars. It's supposed to take around 9 bars of metal to forge a breastplate, but the raws say 9. A bar is 150 units, so if you put 1,350 as the requirement for a breastplate, it will consume 9 bars. It's just a rounding issue. That leftover 141 from a non-edited creation is abstracted away. Similarly with soap, every bathing uses up less than a whole bar. Toady changed how metal works when he added soap as slowly-used, but he never updated the forges to reflect this change. When it gets fixed, warfare is going to change drastically.
As for other items, I think eventually you'll see stack numbers used on singular items. For instance, you might kill a kitten and get cat leather[4]. A cloak would require [10] but mittens require [2], so you could make 2 pairs of mittens but not a cloak. If a whale gave you leather[200] then you could make 20 cloaks, or just make one cloak and have leather[190] left in the workshop. This might add some annoying data to the already-existing FPS drag, but more than likely would just cause slightly larger save files instead of lowered fps. It would also allow things like log[50] and earring only use [1], so you can mass produce small crafts. Or a wooden spear is [10] so cheap weapons are easier. It would change a lot of how production works.
Granted, this is my idea. No idea if Toady will implement this or not, or when.
As for leather, you can edit it slightly. There's no butcher return for skin, which makes it unique. Muscle has a unique butcher return, labeled as "meat", lungs have a special return labeled as "prepared lung". Skin has no such name, and this, apparently, makes it only return 1 item. You may be able to declare skin as "skin" and make it produce stacks, but the workshop is similar to the bonecrafter, it tends to take a whole stack all at once. So even if you got Whale Skin[400] the workshop might only produce 1 leather from 1 stack. Or you could try to make a custom reaction for tanning hides, and attempt to split stacks into multiple leathers. I think there's a way to specify returns like that.