You can have more then 8 rows displayed in Excel 2003.
The way to create that option is different depending on which controlbox you used.
If you used the Controlbox from the Forms toolbox, then right-click the controlbox and select Format Control, then in the Control tab, adjust the Drop down lines field.
If you used the ControlBox from the the Control Toolbox, then go into Design mode and right click the controlbox, select Properties and adjust the ListRows field