The rears (as the other members have said) are generic 6x9.
The front speakers are 6.5 but you will require the use of an adapter. Some have used the stock speaker surrounds as the adapter but I found a much better method.
I purchase these adapters each time I retrofit my SVX with new speakers. Some brands of after market speakers may call for these adapters to be trimmed but this can be done simply with a utility knife, as these are made of a soft plastic/mdf-like material. I find these adapters and spade connections make for the cleanest, simplest, and most suitable way to install front after market speakers.
The ebay link is
HERE.
I have bought five sets of these adapters so far for all of the SVXs I have but speakers in.
Also, I HIGHLY recommend the MB Quart products.
http://cgi.ebay.com/4-NEW-MB-QUART-S...3A1%7C294%3A50
All four speakers for $148!
I personally have used the DKE-116 for the front, and DKE-169 for the rears.
I have retrofitted these to four SVXs so far with top-notch results! They are absolutely the best speaker value on the market today.