What are story points and T-shirt sizing?
First things first, a team should choose the unit of measure for their sprint planning meetings. Story points and T-shirt sizes are two popular choices in Agile planning. These units, unlike hours and days, come from an understanding how much effort is required to implement a story or an epic task.
At first glance, these measure units may seem a little tricky to understand. We are used to hours and days in everything. But to presume that a story point equals one hour would be wrong. It would undermine the very idea of a story point.
When planning in story points, it's not about how much time we assume it may take to finish this story. Instead, it's about what activities it will take to complete the story and how complex we as a team with our current knowledge think these activities are. Then we estimate the story relative to a previously defined baseline story.
It means that one and the same story does not necessarily get the same estimate by two different teams. It all depends on the teams' experience, knowledge, and skills. For instance, the task is to paint a room. One team could give 1 story point to the painting of one of the walls without a window. The same team could give 0.5 story points to the painting of a wall with a window, since there is less surface to be painted. However, another team could give 2 story points to the painting of a wall with a window because they might not have the tools necessary to cover the window while painting. It's more complex and requires more effort from their point of view.
You might go for T-shirt sizing for estimation if your team is not very familiar with the concept of story points. Every T-shirt size stands for the amount of work needed to be done to complete a task. Just like story points, T-shirt sizes do not equal hours or days.
To kick things off, you can take a story (a small one) that you've implemented previously and make sure that everyone on the team understands what activities and requirements it entailed. It is important to be on the same page regarding this baseline story. Then decide how many story points or what T-shirt size your team agrees to give this story, and you have on your hands a sample estimate to guide you when estimating other stories.