Size is working properly. It is telling you that there are TWELVE characters in the string 'flights_2011'.
size(flights_2011) will return [1 35040]
35040 is the number of columns. Excel has 16384 IIRC, and it therefore won't accept 35040 columns of data.
If you simply transpose flights_2011, it'll fill 35040 of excel 95's 65536 rows