Yeah, radiator size doesn't matter if the Tstat is properly working. Radiator size doesn't matter if the Tstat is working, because you can't cool fluid that doesn't get to the radiator. I would believe that the Tstat could be bad if you had a minor change when you changed the radiator, but not that big. You should still come off of cold with the car idling for a while in the driveway and the fan off, no matter what.

Don't jump to conclusions and assume it's the thermostat though, the engine could be working just fine and the gauge could be wrong. On most cars this generally means that the ECU thinks you're running cold also, even though you're actually not.

Drive the car around for a little bit to get it at steady state, and then let it idle for a while. It should come up to some decent temperature (with the fan off) even if the thermostat is stuck open. If this is not the case, you may have a coolant temp sensor problem. That is very common also. You can find troubleshooting procedures in a a factory manual, and most likely online too without much trouble. You just need a multimeter, and the testing is very easy.

Just out of curiosity, was there any wiring modifications involved with the new fan?

Thermostats and coolant temp sensors are both cheap and easy to replace though, so get to troubleshooting.