Final answer:
The 'Number of energy levels' is most important for determining the size of an orbital, as represented by the principal quantum number, n, which indicates larger and farther orbitals from the nucleus as n increases.
Step-by-step explanation:
The most important information in determining the size of an orbital is 'Number of energy levels'. Each energy level corresponds to a different principal quantum number, denoted as n, which determines the general size and energy of the orbital. Higher values of n indicate orbitals that are farther from the nucleus and larger in size.
The principal quantum number is one of three quantum numbers used to characterize an orbital. It defines the energy of an electron in a hydrogen or hydrogen-like atom or ion and the general region in which discrete energy levels of electrons in a multi-electron atom are located.
An atomic orbital is the space around an atom's nucleus where an electron is most likely to be found. This concept is central to the quantum mechanical model, which uses mathematical wavefunctions to specify the probability of finding an electron within three-dimensional space around the nucleus.
In conclusion, the size of an orbital is determined by the principal quantum number, with higher values of n indicating larger orbitals further away from the nucleus.