I'm looking for speakers for my Aiwa 770 to replace the non-original ones in it now.

They are 6-3/8" (16cm) 4 ohm. Aiwa 600 or 660 speakers would also fit. 

I'm in the USA.

Which Aiwa CS-770 ( Letter designation after the 770 '?' ) do you have? 

Actually the woofers in the CS770,CS660 and CS600 are 6 5/16" (16cm) according to the official Aiwa literature.

It's a 770u.

In my cs660 owner manual they're listed as 6 3/8".  Close enough.