Normally 8k by default.
In 8.3.0, you can change BLCKSZ in src/include/pg_config_manual.h, and recompile from source to change it.
How can you tell what the page size of your installation is? Create a table with a single item in it. Then print the size of the table - since it fits on one page, the size is the size of the page.
_[marcua@sorrel postgres]$ psql -d imdb
Welcome to psql 8.3.0, the PostgreSQL interactive terminal.
Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit
imdb=# select 1 into test;
imdb=# select pg_size_pretty(pg_total___relation_size(‘test’));