I think I've found a less intrusive solution that, furthermore, it doesn't change the width of all your sheets. This way you'll be able to specify which sheets have a non-standard width:

1- Define your own css classes in your own css file this way:

.openerp .oe_form_sheetbg {
  padding: 16px 0;
  max-width: none !important;
}
.openerp .oe_form_sheet_width_wider {
  max-width: 1100px;
}

Note: it is mandatory that you "overwrite" the oe_form_sheetbg class to add the max-width line. This class is applied by default to the div containing the background image. When you add a class or style to a sheet, the background div gets it too and if you change the sheet width, the background width changes too.

2- Put it inside your module (eg: <yourmodule>/static/src/css/mycssfile.css) and load it in __openerp__.py:

'css': ['static/src/css/mycssfile.css'],

3- Add class="oe_form_sheet_width_wider" to the sheets you want to have different width. If you need to modify an existant sheet through heritage, you can do it this way:

<xpath expr="//sheet" position="attributes">
    <attribute name="class">oe_form_sheet_width_wider</attribute>
</xpath>

If you don't want to create a class for every different width you can use style="max-width: ..." in your sheets, but you'll still have to overwrite the oe_form_sheetbg class as mentioned.

I found this link, this should work (unfortunately my karma is too low to post links, but you can copy/paste it in the browser):

Change CSS to Increase Width of Sheet.

Egor

Instead of this:

.openerp .oe_form_sheet_width {
    min-width: 650px;
    max-width: 1260px;
    margin: 0 auto;

I would recommend using something like:

.openerp .oe_form_sheet_width {
    min-width: 50%;
    max-width: 90%;
    margin: 0 auto;

This will automatically set the width to be a pecentage of current window size. The previous answer will make the sheet "too big" sometimes, even when you resize your browser

Hope it helps!