There might be an online resource that gives the volume (in cubic feet or ?).
If you need more precise numbers and can't come across the numbers any other way, another way to get the numbers would be to arrange to view an 03 Boxster and while at the car whip out your tape measure and make some quick measurements.
Oh, one of the USA car mags (R/T, or ?) when it does a full test of a car publishes details about the car and IIRC this includes cargo dimensions. You'd have to find a good source of car mag back issues online to see if you could find the review of the 03 Boxster, though.
Sincerely,
MarcW.