=COUNTA(IF(MASTER!DA:DA='2013 List sizes'!B7,MASTER!DF:DF))

The answer should be 4 but it returns 65536.
1) Your working in Excel 2003?

And

2) MASTER!DA:DA='2013 List sizes'!B7 = empty

then you get the result 65536 (all cells in the column).