How To Set A CSS Width From A Javascript Function Result

I have a GridView in a div wrapper, there is some row headers along the left side that need to be always visible. This all works so far, but I need the wrapper to have a variable width to fit the browser size.

I got the desired width of the wrapper based on the browser width using some javascript but I can't figure out how to set this width as the wrapper.width.It doesn't have to update the wrapper width after the page loads or check for browser resizing.

my poor attempt at diagramming:

| |column headers |
| R |--------------|
| O | gridview data |
| W | |
| | this part |
| H | will scroll |<--->
| E | while the |
| A | Row headers |
| D | stay there |
| E | |
| R |______________ |
| S | scroll bar |


asp:(see edit below)

<pseudocode>
<table>
<tr><td>row headers</td> [code]...

I either need a way to set the wrapper.width = Width or a completely different and hopefully better way to achieve this.

I tried using a % for the width but it doesn't use the browser window as 100%, it takes the % of the full width of the whole GridView

which does nothing for me.

Edit: added some code

<script type="text/javascript">
var Width = window.innerWidth - 275 || document.body.clientWidth - 275;
var divElement = document.getElementById("wrappist");[code]....