Lotus eyes are simply large, beautiful eyes, often compared to the petals of a lotus flower due to their size and perceived beauty. This is primarily a poetic or literary description, not a literal anatomical term.
The reference provided illustrates this well. In the context of Viswamitra describing Rama and Lakshmana to Janaka, "lotus eyes" isn't a clinical description but a way to convey the striking and attractive nature of their eyes. The comparison draws on the lotus flower's cultural significance as a symbol of purity, beauty, and enlightenment.
Therefore, when someone is said to have "lotus eyes," it means they possess large, beautiful, and captivating eyes that are pleasing to the observer.