[pbhjpbhj]

Small, medium, and large are contextual. If you lack sufficient context to use such classifiers then objective terms can be used - '[a heap of] 200g of wheat'.

Personally I'd classify "any quantity of discrete articles in which are least one article is held up from contact with the ground/container as being a heap". Heaps are haphazard stacks to me; ergo 2+ items constitute a heap when they are arranged appropriately.

A heap of grain is no longer a heap when spread across furrows (but may be lots of little heaps!).

It's like temperature, a volume of stuff being hot is variable 1K is hot on some circumstances, 10E3 Kelvin if cold in others but we have absolute scales to work around such issues.