Where do I set board width

[Rich]

How do I make a fluid board a fixed size and whats a good fixed size?
Thank you in advanced

 

[Paul B]

See point 2 on this thread for how to set the width: http://xenforo.com/community/threads/how-do-i.5183/

The size you choose depends on a few factors.
Most people however ensure it will fit on a 1024px wide monitor so the maximum you should go is around 980px.