Answer:
Most non-vascular plants are small because they can only transport water short distances.
Explanation:
Vascular systems in plants enable the water to circulate higher in the plant. Without it, the plant has no way of getting the water to it's higher parts.
For cells that are larger, nutrient intake and waste elimination takes a longer time due to the larger distance that must be covered by the same transport processes as the ones taking place in smaller cells. Shapes that maximize surface area help increase the rate of nutrient uptake and waste elimination.