Enlarge popup

private FieldInfo _popupSearchFormFieldInfo;

        internal FieldInfo PopupSearchFormFieldInfo
        {
            get { return _popupSearchFormFieldInfo ?? (_popupSearchFormFieldInfo = GetPopupSearchFormFieldInfo()); }
        }

public void SomeChooseBox_TextChanged(object sender, EventArgs e)
        {
            var cb = sender as MultiChooseBox;
            if (cb != null && cb.Text.Trim().Length >= cb.SearchIndex)
            {
                var popupSearchForm = PopupSearchFormFieldInfo.GetValue(cb);

                var searchPopup = popupSearchForm as PopupBaseSizeableForm;
                if (searchPopup != null)
                {
                    searchPopup.MaximumSize = new Size(400, 255);
                    searchPopup.MaximumClientSize = new Size(400, 255);
                    searchPopup.ClientSize = new Size(400, 255);
                }
            }
        }

private FieldInfo GetPopupSearchFormFieldInfo()
        {
            var fields = GetAllFields(typeof(MultiChooseBox));
            FieldInfo fi = fields.Where(i => i.Name == "searchForm").Select(i => i).FirstOrDefault();
            if (fi == null) return null;
            return fi;
        }

private IEnumerable<FieldInfo> GetAllFields(Type t)
        {
            if (t == null)
                return Enumerable.Empty<FieldInfo>();

            BindingFlags flags = BindingFlags.Public | BindingFlags.NonPublic |
                                 BindingFlags.Static | BindingFlags.Instance |
                                 BindingFlags.DeclaredOnly;
            return t.GetFields(flags).Concat(GetAllFields(t.BaseType));
        }