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You should treat the event QueryPopup and modify the popup size by assigning a Size with the values you wish: 
private void gridLookUpEdit1_QueryPopUp(object sender, CancelEventArgs e) {
    GridLookUpEdit editor = (GridLookUpEdit)sender;
    RepositoryItemGridLookUpEdit properties = editor.Properties;
    // mudando apenas a largura, mas não a altura, por exemplo:
    properties.PopupFormSize = new Size(500, properties.PopupFormSize.Height);
}
