Notes

T-shirt sizing is an estimation technique based on effort. It is a very popular estimation technique in agile mindset-based methodologies. It is a relative estimation technique. This type of estimation is generally performed collaboratively as a team where the team agrees to the size, while providing input on their effort in their role. The product owner may then add up the efforts and assign a T-shirt size.

In this technique, items are classified into t-shirt sizes: XS, S, M, L, and XL. Each size represents a range of effort. To perform T-shirt sizing, the following steps are performed:

Make a list of the stories to be estimated.
Ask the team to provide effort estimations for each story. Consider factors, such as complexity, value, risk, dependency, etc. when performing estimations.
Now, sort the stories, based on their relative size. For example, look at Story 1 and 2, then 3 and 4, and so on. Assign story efforts relative to each other. Adjust estimations as needed.
Based on the above, establish a T-shirt sizes evaluation criteria or a range of effort.
For example,
XS = 1 day worth of effort
S = 2 day worth of effort
M = 3 day worth of effort
L = 5 day worth of effort
Alternatively,
Small = < 4 hours of effort
Medium = 1-2 days
Large = 1 week or more.