A chunk as minimum material requirement sounds rather much, but then again, You don't need tools like the hammer in large quantities; And in a sense, they are very powerful.
But I'm not a fan of having different resources being handled differently. "ok this is a large metal beam, but because I use it to build tools and it has one production tier more, I can carry it, but I can't easily carry the rock, because.. uh.."

Just a though here: We also might consider making the "small objects" a unit. So a chunk of rock would still be "worth" "5 Stones", but could not actually be split into objects. But feeding it into a production building would add "5 stone" to its available production material. It's very similar to how gold works in Clonk Rage: "Object-Gold" is worth 5 "Money-Gold". Getting those out of production buildings again might be tricky, but in the first stage we can simply not allow it. I would then apply this to all the resources we have. So a wooden beam is worth 5 wood, a metal beam is worth 5 metal, etc pp. this would allow for "smaller" production requirements.