what does an eye size of 50 mean. My PD is 27.5 and 19.5 how do I enter that correctly?

The eye size 50mm is the lens width measurement. To make sure the frame you choose will fit you well, the most important measurements you should take care are the total width( the distance from temple to temple)and temple length. By selecting frame with similar measurement, it will fit you perfectly.
We always recommend our customers select the frame size similar to their existing ones to achieve the most comfortable wear.