spender

I am going to upgrade the stereo in my 2005 Boxster with a third party (non-Porsche) headunit with a Double DIN dashkit. Are all "Double DIN" radios the same size? If not, what is the largest dimension that will fit into the dash in a 2005 Boxster?

12vElectronic

Yes, double din head units are standardized in size. They are 4" tall.
I recommend sticking with a brand name unit. The top five brands are Alpine, Sony, Kenwood, JVC and Pioneer.