How do I use the DB Forge to create a DECIMAL data type?

[eluser]Citizen[/eluser]
I'm looking to store lat and longitude values. Here's where I'm stuck:

Code:

'user_id' => array(
'type' => 'DECIMAL',
'constraint' => ?,
'unsigned' => FALSE,
),

Constraint is an integer, and I need my type to be DECIMAL(9,6)

Any ideas?