postgres teoretical and (best-)pratical size limits

I have some questions about the subject.

in the postgresql F.A.Q.:
http://www.postgresql.org/docs/faqs.FAQ.html#4.4

"...
4.4) What is the maximum size for a row, a table, and a database?

These are the limits:

Maximum size for a database? unlimited (32 TB databases exist)
Maximum size for a table? 32 TB
Maximum size for a row? 1.6TB
Maximum size for a field? 1 GB
Maximum number of rows in a table? unlimited
Maximum number of columns in a table? 250-1600 depending on
column types
Maximum number of indexes on a table? unlimited
..."

1) How are computed the maximum size of a table?

On the manual:
http://www.postgresql.org/docs/8.0/interactive/sql-createtable.html

"...
A table cannot have more than 1600 columns. (In practice, the effective
limit is lower because of tuple-length constraints.)
..."

2) What's the tuple-length constraint? (and what's the max tuple-length?)

3) These are teoretical limits, but exists some licterature (articles,
examples, case-studies) what addresses on pratical performance
degradation derived from use of large tables with large amount of data?
(eg.: max tables size suggested, max rows, max tuple size...)

Thanks for the answers and best regards.