Problem with fontsize when using longtable

\fontsize is defined as:

\fontsize{size}{skip}

The two arguments to \fontsize are the actual font size in size and the size of the baseline-skip in skip. The baseline-skip should be set to roughly 1.2x the font size. Reference

The declaration

\fontsize{9}{11}\selectfont

affects all following text until the end of the current group, the fact that there is a longtable there isn't that relevant. You can use \normalsize to reset it or use { and } to limit the scope. It really is better to use \small or other named sizes to get consistent sizing rather than setting each table to individually set point sizes. The scoping rules would apply to \small as well.

Enclose the longtable within \begin{small} and \end{small}, this will ensure the text to be of small size. Worked well for me. Or start the longtable with \small, ending it with \normalsize where the table ends.