I just replaced my wipers with PIAAs...
I found it quiet difficult to find the right ones... size and curved/straight.
The PIAA UK Site recommends the wrong sizes 22". The US PIAA site does a better job explaining the differences and recommends
Driver Side 18"
Passenger Side 15"
Back N/A
But still not sure if any should be curved or all straight are fine.
http://www.piaa.com/controller.php?action=getWipers&make=PORSCHE&model=1631
I chose the following based on the size of the existing wipers I was replacing which seemed to work well so I figured still with same sizes.
17" Driver Side
16" Passenger Side
15" Back
So here is what I found when installing them.
Back Wiper
For the back wiper the clips supplied are compatible with the wiper arm, and the PIAA wiper does not fit using the exiting clip from the existing wiper... so what you can do is remove the rubber blade from the exiting wiper and replace it with the rubber blade from the PIAA wiper... 15" is the right size and all works well.. If you can buy just the rubber blade instead of a full 15" PIAA wiper you will save your self some cash.. but I have not see where you can buy just the rubber blade part.
Front Wipers
Firstly the instructions recommend using clip C3 for Porsche... but you should use C1 the smaller clip.
Drivers Side.
17" seems to work perfectly, except for the overhang at resting position. This makes me wonder if curved rather than straight might be better choice for the Driver Side... not sure if any one has tried that version. The suggested 18" might also work.. but my 17" wiper seems to go right to the top of the screen so I'd be concerned that the 18" is too long???
Passenger Side
16" are a little too long... it wipes over the rubber at the top of the windscreen by a half and inch, which you wouldn't think would be a big problem but what it does is affects the half an inch of the rubber blade so it leaves a slight trail of water. I reckon this would not happen with 15" blades.
Pics below of what I found