1st gen S series radiator ???

Well my radiator took a shit on me the other day. When I called to order a new one I was asked what size it was. From what I was told there are 2 different sized radiators these trucks have(I a not sure what they are atm). The only problem I have is the radiator that is in the truck now was put in not long before I purchased the truck and it is used.I am not sure if it is the right one to begin with. Does the radiator size matter with these? Or do I have to know the exact one? And if it has to be the exact one how do I know what size I will need?

My truck is an 89 GMC S-15 with a 4 cyl with a standard tranny and no AC.

yea they are different. I know the 4.3 and 2.8 ports are swapped. I'm not sure on the 4 cyl. though.