To estimate the size of an epic, you can use “plan estimate” field in Epic Detailed View in order to estimate Epic. This is a numeric value that can be referred to as reference size value to another epic size.
Note: Epic Plan Estimate value is independent of Project Settings.
User Story estimation methods are different than an Epic estimate. You can choose among different supported estimation methods for user stories. These Estimations are also visible on Epics Board, for each User Story associated to an epic. An Epic progress percentage is also dependent on the progress of its underlying User Stories.