You will need to use the original expression in a conditional then. Say Count(FileNo) is your original expression that returns 1 or 0, and that the expression that you want to use is Sum(ByteSize) if that expression equals to 1:
If(Count(FileNo) = 1, Sum(ByteSize))
The If() will slow your chart considerably though, so think on that flag field or something to improve performance if you have some thousands of rows.
Hope that helps.