Commodities are the actual things that are traded between worlds. We could keep things very simple, but part of the purpose of this is to provide background for RPGs, so it's far more interesting for there to be a wide range of commodities for PCs to have available for buying and selling.
The larger the resource value, often the bigger the creatures. Note that this doesn't apply to resources which are defined by being small (prokaryotes and metazoa for example). Exact sizes will depend on the type of creature.
On most worlds, life evolves in the oceans so early animals will be found there. They may evolve into land variants, which will be a different resource type.