I thought I remembered this one, from SQL Server Magazine UPDATE, May 3 2001...
* TIP: MAXIMUM ROW SIZE
(contributed by Richard Waymire, rwaymi@microsoft.com)
Q. How can I find out the maximum row size for a table in which all the
varchars are fully populated?
A. The only way to find a SQL Server table's maximum row size is to
query the system tables (or the INFORMATION_SCHEMA views) and add up the
byte counts. But note that Microsoft doesn't recommend querying system
tables, which can change from release to release. Also note that the
total row size never exceeds about 8060 bytes in SQL Server 2000 or
7.0.