To be able to understand how the size of a molecule is related to its biological function, you must first know the concept of a lock and key. When the key does not match the lock, then you are unable to open the lock. This goes too with molecules in the body; they should conform to the shape needed by the cell or tissue for it to perform its biological function.
The correct molecule’s shape would activate the receptor’s site
A good example is an endorphin, which "fits perfectly into the space provided in the receptor molecule." If the endorphin’s shape does not fit exactly like a key, then its combination with the receptor would not be able to result in any biological reaction. Another example is a substrate. When the substrate molecules’ shape does not fit perfectly into the active site of the receiving enzyme, then the lock (enzyme) would not open, hence, they would be unable to perform their biological function. Structures and shapes of RNA also affect its effectiveness to accomplish its functions. If they are single-stranded or loop-shaped, this would affect how they react with other strands.
The molecule’s shape is instrumental for the movement of molecules
A molecule’s biological function could not take effect if its shape did not allow it to be transported to the cell where it is needed. Larger molecules are not transported easily because they could not pass easily through the pores of cell membranes. Smaller molecules could pass easily from one cell to another. Hence, substances which are commonly needed in most tissues in the body are usually smaller in size than those which are needed rarely. This is the reason why the biological functions in the body do not get mixed up.
The biological function of a molecule is also related to its shape because dense and round shaped molecules are generally connected to transport functions, such as transport of oxygen and hemoglobin by the cellular elements of the body called red blood cells. Different shapes of proteins also have specified functions in the body. Polysaccharides come in different sizes and shapes based on their biological functions in the body.
Even in vitro, there is a significant relationship between function and shape. Whatever the function of the molecule, the shape has to adapt to the biochemical reaction that the molecule is tasked to do. You can always recall the lock-and-key concept to understand this process.