Querying the schema for column widths - what syntax do I use?

I am working on some upgrades to the MyDNS open source product. I have
some expertise in MySQL but am not overly familiar with PostgreSQL and
need some guidance on how to query the schema for the maximum size of
data a column can hold.

In MySQL I can do either: a "DESCRIBE" command or do "SELECT COLUMNS"
command. And then parse the result for the length in the type column of
the row returned. How would I do a similar function using PostgreSQL - I
have tried to find this in the manuals and in this mailing list but not
found any pointers to get me started.

Apologies for asking such a simple question but I am being a bit lazy as
I want to get on with releasing the MyDNS code.

Regards, Howard.