What is the correct term of a chemical formula for very large molecules if not just "chemical formula"?
Chemical formulas of the type you describe are just as valid for large molecules as for small ones. The reason they are not commonly used is that the number of possible molecules described by a given formula goes up exponentially with the number of atoms. 

Thus, it is much more common to describe larger molecules with more precise terms. In the case of polymers like proteins or nucleic acids, the primary sequence of the constituents (amino acids or bases) is often used if space allows. Alternatively, a protein or nucleic acid  might be referred to by a common name with which a sequence is associated.

Even for small molecules, IUPAC formal names or common names are used in preference to molecular formula in many cases, because the names provide information about the atom connectivity that is absent in the molecular formula. Other representations that include connectivity information are forms such as SMILES, which is a standardized system for representing connectivity in text. 

In the scientific literature, names of molecules are often accompanied by pictorial representations that provide atom connectivity information in an easily understood format. For very large molecules like proteins whose connectivity cannot be easily represented in a small image, pictorial representations are more often used to show secondary and tertiary structural features.