Assuming the column of shutter widths is on Sheet2, column B, you can enter this formula in cell E5:
=INDEX(Sheet2!B:B,IF(ISNA(MATCH(C5,Sheet2!B:B)),2,MATCH(C5,Sheet2!B:B)+IF(ISNA(MATCH(C5,Sheet2!B:B,0)),1,0)),1)
There may be a shorter formula, but this should work, assuming the "MASONRY OPENING WIDTH" column heading is on row 1, and the data starts in row 2.

HTH

Jason