Holy cow, I wrote a book!
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.