There is variable $sheet-max-width in the base.sass of /web. BUt Dont know how to recompile the CSS from the sass

You can find how to convert SASS to CSS : www.vionblog.com/modify-openerp-7-web-client-style/

One Simple Way is To add a Div to your whole Form view. For Eg:

let this be a form view..

<form string="Export Data" version="7.0" >
*** form Contents***
</form>

you could add a Div as shown below

 <form string="Export Data" version="7.0" >
      <div style="width: 300px;">

        *** form Contents***

       </div>
    </form>

After this restart the server and view your form view. U will see the difference. Then change the value from 300px to some 1500px and restart the server and view the form again. And See the differnce