Series 390 Channel Balances

I replaced several of these in a rental house but the ones for the small window above the tub were missing altogether. I think I need a 16" or a 17" based on measurements of the balances and springs for the larger windows in the house. These are vinyl doubel pane windows. The sash for the small window is 18" high on one side (front or back, I don't remember which) and 17 3/34" on the other side. Which balance do I need?