I've looked at a number of sites and keep getting different sizes, I don't know what's what, please help.

I will see if I can find out, Did your car come with no tires?

Looks like 15 X 4 X 100.
Tire Size/Wheel FAQ - Wheel and Tire Forum - j-body.org - the jbody organization (http://www.j-body.org/forums/read.php?f=42&i=21297&t=12684)
then scroll down to the Plymouth Horizon.