I own a 1993 Japanese model Nissan Prairie and want to replace the
factory speakers with aftermarket speakers. Does anyone know what size
front and rear speakers I need, and models that will fit
Japanese-built cars?

I tried looking up data in 1993 speaker brochures (yes I'm sad enough
to have a few Alpine, Kenwood and Philips brochures for 1990-95)
Unfortunately your car is not listed, probably as it is not a saloon
car. I have also looked online trying to source an online speaker
selector. One easy way of determining the speaker size is to take them
out yourself and measure them, (that is if you are purchasing and
replacing the factory replacement speakers yourself... I assume so?)
From experience the speaker manufactuer guides are for the speaker
size only, sometimes the mounting is different or the holes in their
speaker are in a different location to the holes in your doors/dash.
The safest solution I have found is to take the speakers out and
either take the speaker to the shop or make a note of the screw hole
positions.
I hope that helps
lot-ga

Its odd, Crutchfield claims the 
93 Nissan Quest uses 6x8 speakers.
The Quest seems the most likely
US model to match the Prairie.

You can call Crutchfield 8am-midnite
they say. Everyday?
http://www.crutchfield.com/S-BTYKiLnZjvt/advisor.html
Call 1-888-955-6000

They will be able to hook you up,
or at least let you know which way
to go with your vehicle.

-AI