The team estimates the effort required for each story using one of a number of different methods. Generally, stories are compared with other stories to understand their sizes. It’s common to take an average story from the sprint backlog, assign it a 5 or medium and size all other stories against that. It’s not important what number or value you give to a story but we need to measure the effort. When you are estimating stories in the next sprint it’s worth taking a completed story from the previous sprint and using that as a size guide. Over time your estimates will become more accurate, particularly if the delivery team remains the same.