Larry Osterman discussed the buzzword T-shirt sizing, which means “making extremely rough estimates in terms of a small number of predefined categories.” The term comes from the traditional way T-shirt sizes are specified in the United States. Instead of having T-shirts in sizes 4, 5, 6 and so on, there are only a small number of sizes: Small, medium, large, and extra-large. (Sometimes augmented by extra-small, extra-extra-large, etc.)
Forcing the estimate into one of a fixed set of sizes allows the process to go quickly while still producing a result that is at least within the same zip code as a more detailed answer. The idea is not to pin down the schedule precisely but rather to get a back-of-the-envelope feel for whether a project is a two-week project, a two-month project, or a two-year project.
The size breakdowns vary depending on the scope of the project. For a small project, small might mean “less than a day”, whereas for a large project, small might mean “less than a week”. People can usually come up with a gut feeling as to whether something will take “less than a week” much more quickly than they can say whether something will take one day, two days, or three days.
As the concept of T-shirt sizing spread through Microsoft, it inevitably became the source of a new set of derived jargon. I’ve seen T-shirt costing on a meeting agenda. Though thankfully I have yet to see T-shirting.