Why do you think nonvascular plants cannot grow very large or tall?

Nonvascular plants are those that do not have any xylem or phloem. That is to say, they do not have any vascular system. The vascular system is responsible for the transport of nutrients and water to various parts of the plant. Thus, the vascular system can be thought of as the nutrient transport system of a plant. In the absence of such a system, the plant can only absorb water and nutrients from its immediate surroundings without the ability to disperse the fluids throughout the plant. The movement of water in such plants occurs through the process of diffusion. This process is slow and thus would be unable to supply water to all the different parts of a tall or large plant. Thus, nonvascular plants are generally smaller and shorter, as compared to vascular plants (that utilize the xylem and phloem for water and nutrient uptake and transport).

Hope this helps.