Question

Why are nonvascular plants small

Answer 1

One vital part of sustaining a large multicellular organism is the need for nutrition (water, minerals, energy) to be readily available to every cell. Plants typically solve this by what a vascular system, or a network of tissues that specialize in transporting nutrition to where they're needed. This tissues from long tubes that run up and down the plant. It runs water and minerals from the roots to up the plant, and runs sugar generated by the leaves down the plant. This means that large plants can compile all their resources together and send it throughout the cells.

Nonvasular plants lack this system, so the individual cells have to be responsible for making sure nutrition get to where it needs to be. This would make a large one impossible to maintain, given how naive transportion would be.