The converted PDF's page size is not the same as the size specified in "General Settings > Page Size".
The application might decide which is the best page layout for you, e.g. Microsoft Word. As a result, Gaaiho PDF Driver concurs with the application's setting. In other words, even if A4 is specified in printer properties, Gaaiho PDF driver would create PDF according to Microsoft Word's "Letter" size.
If you are using Office 2003, under Tools > Options, choose "Print" and uncheck "Allow A4/Letter Resizing".
If you are using Office 2007/2010, open "Word Options" from the left-upper corner menu, in "Advanced", choose "Print", uncheck "Scale content for A4 or 8.5 x 11" paper sizes".