Thank you for writing.
I assume that you are asking about the filter button located in the grid's header cells, since the one located in the filtering cell will not be visible when you hide the row. To increase the size of the button image, just set an image with the desired size in the ViewCellFormatting event:
sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
GridHeaderCellElement headerCell = e.CellElement
headerCell.FilterButton.Image = Image.FromFile(@
I hope this helps.
the Telerik team