1999 Daewoo Nubira losing water for New radiator could it be too small?

Well short answer is NO and long answer is HELL NO. If it is loosing water it must be leaking from somewhere. Have you checked the hoses for holes this is usually where a leak happens ( if a hose has a hole it cant hold high pressure ). alliteratively try the head gasket ( yes i said head because it is the leader of all gaskets ) Although you might want to try replacing the radiator cap or the welsh plugs( yes i said welsh and no they arent british ay govenor). Or just put the radiator cap back on you f@$%*ng nube.

But if the new radiator is smaller than the original you got shafted