The simplest way to control the width, overall, is to place the forum control inside a table (1 row, 3 columns with the contol in the center column) then style the table to whatever you need.
You can do it using a DIV wrapper, just make sure you deal with cross browser issues for scaling. This can be difficult, hence the table approach.
For an example, have a look at www.djdiscoveryworld.com (click on the forum). Then just view source.
HTH
:)
.....the man in black fled across the desert..........and the gunslinger followed.....