You can use code similar to the code below to set these colwidths.
int width = (grid.ClientSize.Width - grid.Model.ColWidths[0]) / grid.Model.ColCount;
for(int i = 1; i <= grid.Model.ColCount; ++i)
{
grid.Model.ColWidths[i] = width;
}
Or if you want to do it dynamically through an event, you can use the grid.Model.QueryColWidth event. There if e.Index > 0, you set e.Size = width (as calculated above) and e.Handled = true. You can see code similar to this in the \Essential Studio\3.3.0.0\Windows\Grid.Windows\Samples\DataBound\GridDataBoundImageCell sample.