C# - Unable To Set GridView Row Widths On RowDataBound?

My current code to attempt to set widths for the rows on a gridview is:

protected void RowDataBound(Object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.Header)
{
foreach (DataControlFieldHeaderCell dcfhc in e.Row.Cells)
{
dcfhc.Width = 100;
}
}
else
{
foreach (DataControlFieldCell dcfc in e.Row.Cells)
{
dcfc.Width = 100;
}
}
}

But that just keep all of the widths default! What am I doing wrong?

EDIT: I have already tried thise code as well!
PoolToDBHeaders.DataSource = new DataView(headerTable);
PoolToDBHeaders.DataBind();
foreach (DataControlField dcf in PoolToDBHeaders.Columns)
{
dcf.ItemStyle.Width = 100;
}
PoolToDBHeaders.DataBind();